feat(providers): an OpenAI-compatible preset for any vLLM, LM Studio or gateway URL

Step 8 of the provider refactor plan, a new provider made of one record.
'custom' runs on @ai-sdk/openai-compatible with <think> extraction, merges
system messages (open-source chat templates reject several), has no default
URL (so a key alone does not count as connected, and the chat endpoint is
shown under the URL field) and lists models from its /models. Server side:
CUSTOM_API_KEY and CUSTOM_BASE_URL, ADMIN_CUSTOM_* in the admin panel.
env.example and the three provider docs gained a section; the picker count
in the e2e test is 25 and a new e2e test walks through the preset. The
behavior snapshot gained the new provider's 146 rows; its two 'No AI provider
configured' rows list CUSTOM_API_KEY now.

tests/e2e/settings-providers.spec.ts

@@ -127,7 +127,7 @@ test("the provider picker searches, marks added ones, and Enter picks", async ({
         dialog.getByTestId("pick-provider-anthropic"),
     ).not.toContainText("Added")
     await expect(dialog.locator('[data-testid^="pick-provider-"]')).toHaveCount(
-        24,
+        25,
     )
 
     await search.fill("zzz")
@@ -596,6 +596,34 @@ test("Azure needs its resource URL before it counts as connected", async ({
     await expect(dialog.getByTestId("test-models")).toBeEnabled()
 })
 
+test("the OpenAI-compatible preset takes any URL and shows where requests go", async ({
+    page,
+}) => {
+    await openApp(page)
+    await openSettings(page)
+    const dialog = settingsDialog(page)
+    // Not among the popular ones: it is in the full picker
+    await dialog.getByRole("button", { name: /^See all \d+ providers/ }).click()
+    await dialog.getByTestId("pick-provider-custom").click()
+    await expect(
+        dialog.getByRole("heading", { name: "OpenAI-compatible", exact: true }),
+    ).toBeVisible()
+    // No URL yet: a key alone does not count as connected
+    await dialog.locator("#api-key").fill("any")
+    await expect(dialog.getByTestId("step-1-todo")).toBeVisible()
+    await expect(
+        dialog.getByText("Fill in the API key and the base URL first."),
+    ).toBeVisible()
+    // Its options start open; the URL field shows the chat endpoint
+    await dialog.locator("#base-url").fill("http://127.0.0.1:1234/v1")
+    await expect(
+        dialog.getByText(
+            "Requests go to http://127.0.0.1:1234/v1/chat/completions",
+        ),
+    ).toBeVisible()
+    await expect(dialog.getByTestId("step-1-done")).toBeVisible()
+})
+
 test("a model id typed for one provider does not follow to the next", async ({
     page,
 }) => {