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.

app/api/provider-models/route.ts

@@ -6,7 +6,7 @@ import {
     listProviderModels,
     ModelListError,
 } from "@/lib/provider-models"
-import { PROVIDER_NAMES, PROVIDERS, type ProviderName } from "@/lib/providers"
+import { PROVIDERS, type ProviderName } from "@/lib/providers"
 import {
     allowPrivateUrls,
     isPrivateUrl,
@@ -16,11 +16,6 @@ import {
 
 export const runtime = "nodejs"
 
-// Public lists need no key
-const NO_KEY_NEEDED = new Set<ProviderName>(
-    PROVIDER_NAMES.filter((p) => PROVIDERS[p].listNeedsNoKey),
-)
-
 /**
  * The models a provider offers, for the "Fetch models" button in model
  * settings. Answers { models: null } for providers that cannot list them,
@@ -45,7 +40,8 @@ export async function POST(req: Request) {
     if (baseUrl && !allowPrivateUrls() && (await isPrivateUrl(baseUrl))) {
         return NextResponse.json({ error: "Invalid base URL" }, { status: 400 })
     }
-    if (!apiKey && !NO_KEY_NEEDED.has(provider)) {
+    // Public and local lists need no key
+    if (!apiKey && !PROVIDERS[provider].listNeedsNoKey) {
         return NextResponse.json(
             { error: "API key is required" },
             { status: 400 },