refactor(providers): one place for env var names and the server credential check

Step 3 of the provider refactor plan. envVarsOf(provider) spells the key and
base URL variable names from the record's envPrefix and keyEnv; the three
copies of the gateway/AI_GATEWAY spelling (getAIModel's default branch,
globalBaseUrl, the admin panel's credEnvNames) read it. serverCredentialError
answers why the server lacks a provider's credentials, or null; detectProvider
asks for null and validateProviderCredentials throws the text, so the
Anthropic auth token and the Azure URL-or-resource rules exist once. The four
error texts and their order are unchanged (tests/unit/server-credentials.test.ts
was written against the old code first). credEnvNames reads fixedCredentials
and envPrefix and is exported, with a table test over all 24 providers.
Behavior snapshot unchanged.

tests/unit/server-credentials.test.ts

@@ -0,0 +1,168 @@
+/**
+ * The server credential check getAIModel runs when the request brings no
+ * key of its own, and the auto-detection of the server's provider. Written
+ * against the pre-refactor validateProviderCredentials and detectProvider;
+ * the refactor merges both into serverCredentialError, which must keep every
+ * answer and every error text below.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest"
+import { getAIModel, getServerProvider } from "@/lib/ai-providers"
+
+vi.mock("@/lib/admin/settings", () => ({
+    loadSettings: () => ({}),
+    getEnvFallback: (key: string) => process.env[key] ?? null,
+}))
+
+const { factory } = vi.hoisted(() => ({
+    factory: () => (modelId: string) => ({ modelId }),
+}))
+vi.mock("@ai-sdk/openai", () => ({ createOpenAI: factory }))
+vi.mock("@ai-sdk/anthropic", () => ({ createAnthropic: factory }))
+vi.mock("@ai-sdk/azure", () => ({ createAzure: factory }))
+vi.mock("@ai-sdk/google-vertex", () => ({ createVertex: factory }))
+vi.mock("ollama-ai-provider-v2", () => ({ createOllama: factory }))
+
+const VARS = [
+    "AI_PROVIDER",
+    "AI_MODEL",
+    "ANTHROPIC_API_KEY",
+    "ANTHROPIC_AUTH_TOKEN",
+    "AZURE_API_KEY",
+    "AZURE_BASE_URL",
+    "AZURE_RESOURCE_NAME",
+    "OPENAI_API_KEY",
+    "OPENAI_BASE_URL",
+    "GOOGLE_VERTEX_API_KEY",
+    "OLLAMA_API_KEY",
+    "MY_KEY",
+    "MY_KEY_2",
+    "MY_URL",
+]
+
+beforeEach(() => {
+    for (const name of VARS) vi.stubEnv(name, undefined)
+    vi.spyOn(console, "log").mockImplementation(() => {})
+})
+
+const run = (overrides: Parameters<typeof getAIModel>[0]) => () =>
+    getAIModel({ modelId: "m", ...overrides })
+
+describe("server credentials", () => {
+    it("accepts ANTHROPIC_AUTH_TOKEN alone for Anthropic", () => {
+        vi.stubEnv("ANTHROPIC_AUTH_TOKEN", "token")
+        expect(run({ provider: "anthropic" })).not.toThrow()
+        vi.stubEnv("ANTHROPIC_AUTH_TOKEN", undefined)
+        expect(run({ provider: "anthropic" })).toThrow(
+            "Either ANTHROPIC_API_KEY or ANTHROPIC_AUTH_TOKEN environment variable is required for anthropic provider. Please set one in your .env.local file.",
+        )
+    })
+
+    it("holds a server model to its own variable, even for Anthropic", () => {
+        vi.stubEnv("ANTHROPIC_API_KEY", "k")
+        vi.stubEnv("ANTHROPIC_AUTH_TOKEN", "token")
+        expect(run({ provider: "anthropic", apiKeyEnv: "MY_KEY" })).toThrow(
+            "MY_KEY environment variable is required for anthropic provider. Please set it in your .env.local file.",
+        )
+    })
+
+    it("does not fall back to the default variable from an empty custom one", () => {
+        vi.stubEnv("OPENAI_API_KEY", "k")
+        expect(run({ provider: "openai", apiKeyEnv: "MY_KEY" })).toThrow(
+            "MY_KEY environment variable is required for openai provider. Please set it in your .env.local file.",
+        )
+        vi.stubEnv("MY_KEY", "custom")
+        expect(run({ provider: "openai", apiKeyEnv: "MY_KEY" })).not.toThrow()
+    })
+
+    it("needs a URL or a resource name for Azure after the key", () => {
+        expect(run({ provider: "azure" })).toThrow(
+            "AZURE_API_KEY environment variable is required for azure provider. Please set it in your .env.local file.",
+        )
+        vi.stubEnv("AZURE_API_KEY", "k")
+        expect(run({ provider: "azure" })).toThrow(
+            "Azure requires either AZURE_BASE_URL or AZURE_RESOURCE_NAME to be set. Please set one in your .env.local file.",
+        )
+        vi.stubEnv("AZURE_RESOURCE_NAME", "res")
+        expect(run({ provider: "azure" })).not.toThrow()
+    })
+
+    it("lets a server model's own URL variable stand in for Azure's", () => {
+        vi.stubEnv("MY_KEY", "k")
+        vi.stubEnv("MY_URL", "https://res.openai.azure.com/openai")
+        expect(
+            run({
+                provider: "azure",
+                apiKeyEnv: "MY_KEY",
+                baseUrlEnv: "MY_URL",
+            }),
+        ).not.toThrow()
+        vi.stubEnv("MY_URL", undefined)
+        expect(
+            run({
+                provider: "azure",
+                apiKeyEnv: "MY_KEY",
+                baseUrlEnv: "MY_URL",
+            }),
+        ).toThrow(/AZURE_BASE_URL or AZURE_RESOURCE_NAME/)
+    })
+
+    it("passes an array apiKeyEnv on any value and checks nothing else", () => {
+        vi.stubEnv("MY_KEY_2", "k")
+        // No Azure URL anywhere, and the key check still passes
+        expect(
+            run({ provider: "azure", apiKeyEnv: ["MY_KEY", "MY_KEY_2"] }),
+        ).not.toThrow()
+        vi.stubEnv("MY_KEY_2", undefined)
+        expect(
+            run({ provider: "azure", apiKeyEnv: ["MY_KEY", "MY_KEY_2"] }),
+        ).toThrow(
+            "At least one of [MY_KEY, MY_KEY_2] environment variables is required for azure provider. Please set at least one in your .env.local file.",
+        )
+    })
+
+    it("asks nothing of providers without a required key variable", () => {
+        expect(run({ provider: "ollama" })).not.toThrow()
+        // A custom variable is still checked
+        expect(run({ provider: "ollama", apiKeyEnv: "MY_KEY" })).toThrow(
+            "MY_KEY environment variable is required for ollama provider. Please set it in your .env.local file.",
+        )
+    })
+
+    it("names Vertex AI's key variable", () => {
+        expect(run({ provider: "vertexai" })).toThrow(
+            "GOOGLE_VERTEX_API_KEY environment variable is required for vertexai provider. Please set it in your .env.local file.",
+        )
+        vi.stubEnv("GOOGLE_VERTEX_API_KEY", "k")
+        expect(run({ provider: "vertexai" })).not.toThrow()
+    })
+})
+
+describe("server provider auto-detection", () => {
+    it("finds the one provider with a key", () => {
+        vi.stubEnv("GOOGLE_VERTEX_API_KEY", "k")
+        expect(getServerProvider()).toBe("vertexai")
+        expect(run({})).not.toThrow()
+    })
+
+    it("needs AI_PROVIDER when two have keys", () => {
+        vi.stubEnv("GOOGLE_VERTEX_API_KEY", "k")
+        vi.stubEnv("OPENAI_API_KEY", "k")
+        expect(getServerProvider()).toBeNull()
+        expect(run({})).toThrow(
+            "Multiple AI providers configured (openai, vertexai). Please set AI_PROVIDER to specify which one to use.",
+        )
+    })
+
+    it("accepts AI_PROVIDER=ollama with no key at all", () => {
+        vi.stubEnv("AI_PROVIDER", "ollama")
+        expect(getServerProvider()).toBe("ollama")
+        expect(run({})).not.toThrow()
+    })
+
+    it("only counts Azure with a URL or a resource name", () => {
+        vi.stubEnv("AZURE_API_KEY", "k")
+        expect(getServerProvider()).toBeNull()
+        vi.stubEnv("AZURE_RESOURCE_NAME", "res")
+        expect(getServerProvider()).toBe("azure")
+    })
+})