refactor(providers): drop the transitional tables, document how to add a provider

Step 6 of the provider refactor plan. FIXED_CRED_PROVIDERS, PROVIDER_LOGO_MAP,
OPTIONS_OPEN and NO_KEY_NEEDED had no readers left and are gone; the record's
fields are read where they were used. The fixture that compared the derived
tables with their old literal values is gone with them; tests/unit/providers.test.ts
keeps the record's own rules (21 providers in auto-detection, every note in
every UI language, https defaults only, fixed-credential providers, the
picker's order) and a new check that env.example names every record's
variables. docs/{en,cn,ja}/ai-providers.md end with a section on adding a
provider: one record, two env.example lines, one docs paragraph.

lib/types/model-config.ts

@@ -1,6 +1,6 @@
 // Types for multi-provider model configuration
 
-import { PROVIDER_NAMES, PROVIDERS, type ProviderName } from "@/lib/providers"
+import { PROVIDERS, type ProviderName } from "@/lib/providers"
 
 // The provider record, under the name the rest of the app imports it by
 export {
@@ -67,21 +67,6 @@ export interface FlattenedModel {
     isDefault?: boolean
 }
 
-// Providers whose server credentials live in fixed env vars
-// (AWS_ACCESS_KEY_ID, GOOGLE_VERTEX_API_KEY, OLLAMA_API_KEY) with no
-// apiKeyEnv redirection support: their credentials are global
-export const FIXED_CRED_PROVIDERS: ProviderName[] = PROVIDER_NAMES.filter(
-    (p) => PROVIDERS[p].fixedCredentials,
-)
-
-// models.dev logo names that differ from the provider name
-export const PROVIDER_LOGO_MAP: Record<string, string> = Object.fromEntries(
-    PROVIDER_NAMES.flatMap((p) => {
-        const logo = PROVIDERS[p].logo
-        return logo ? [[p, logo]] : []
-    }),
-)
-
 // Suggested models per provider for quick add
 export const SUGGESTED_MODELS: Partial<Record<ProviderName, string[]>> = {
     openai: [