fix(admin): use static error messages in excluded models/providers handlers

[rohith500]

Problem

PUT /admin/v1/excluded-models and PUT /admin/v1/excluded-providers call err.Error() directly when an unknown model or provider name is submitted, leaking the internal Go package name prefix to the client:

{"error":"auth: unknown model id: \"some-model-name\""}
{"error":"auth: unknown provider: \"some-provider-name\""}

The auth: prefix comes from ErrUnknownModel = errors.New("auth: unknown model id") and ErrUnknownProvider = errors.New("auth: unknown provider") in internal/auth/service.go. These sentinel errors are internal bookkeeping — their Error() string was never intended to be user-facing.

Every other 400 response in the admin API uses a static string:

gin.H{"error": "Failed to update excluded models."}
gin.H{"error": "Missing ID."}
gin.H{"error": "Failed to look up API key."}

These two handlers are the only ones in the admin layer that pass err.Error() through to the client on a validation failure.

Fix

Replace the two err.Error() calls with static strings consistent with the rest of the admin API:

// excluded_models.go
- c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+ c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "unknown model id in exclusion list"})
 
// excluded_providers.go
- c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+ c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "unknown provider in exclusion list"})

The underlying errors.Is(err, auth.ErrUnknownModel) / errors.Is(err, auth.ErrUnknownProvider) check is preserved — only the response body changes.

Verification

Before the fix:

PUT /admin/v1/excluded-models    {"excluded":["not-a-real-model"]}
→ {"error":"auth: unknown model id: \"not-a-real-model\""}
 
PUT /admin/v1/excluded-providers {"excluded":["not-a-real-provider"]}
→ {"error":"auth: unknown provider: \"not-a-real-provider\""}

After the fix:

PUT /admin/v1/excluded-models    {"excluded":["not-a-real-model"]}
→ {"error":"unknown model id in exclusion list"}
 
PUT /admin/v1/excluded-providers {"excluded":["not-a-real-provider"]}
→ {"error":"unknown provider in exclusion list"}

make check — all 30 packages pass. fixes #535

[greptile-apps]

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