fix: block global-credential providers already managed via env

Bedrock, Vertex AI, and Ollama credentials live in fixed env vars with
no apiKeyEnv redirection, so a panel instance of one of these would
silently override the credentials that env-configured models rely on.
The API now rejects saving such a provider when the env config already
uses that type, and the Add Provider dropdown disables it with a
'managed via env' note.

app/api/admin/providers/route.ts

@@ -72,8 +72,7 @@ export async function PUT(req: Request) {
     const merged = mergeSecrets(parsed.data, stored)
 
     const envConfig = await loadEnvServerModelsConfig()
-    const envNames = envConfig?.providers.map((p) => p.name) ?? []
-    const validationError = validateAdminProviders(merged, envNames)
+    const validationError = validateAdminProviders(merged, envConfig)
     if (validationError) {
         return Response.json({ error: validationError }, { status: 400 })
     }