fix: repair CI build & lint regressions on main

- openai_gateway_model_availability.go: pass platform through to
  listSchedulableAccounts so OpenAI/Grok diagnosis scopes to the
  correct candidate pool (build break introduced by Grok subscription
  PR #3310).
- no_account_error.go: drop redundant context.Context type on
  ctx := context.Background() to satisfy staticcheck ST1023.

backend/internal/handler/no_account_error.go

@@ -101,7 +101,7 @@ func classifyNoAccountErrorFromGin(
 	displayModel string,
 	platform string,
 ) noAccountErrorClassification {
-	var ctx context.Context = context.Background()
+	ctx := context.Background()
 	if c != nil && c.Request != nil {
 		ctx = c.Request.Context()
 	}

backend/internal/service/openai_gateway_model_availability.go

@@ -6,9 +6,10 @@ import (
 )
 
 // DiagnoseModelAvailabilityForPlatform reports whether the requested model
-// is configured to be served by any OpenAI account in the group. The
-// platform argument is accepted to satisfy ModelAvailabilityDiagnoser but
-// is ignored — OpenAIGatewayService only scans OpenAI accounts.
+// is configured to be served by any OpenAI-compatible account in the group
+// for the given platform (e.g. PlatformOpenAI, PlatformGrok). The platform
+// scopes the candidate pool so distinct OpenAI-compatible platforms do not
+// cross-contaminate diagnosis results.
 //
 // Safe to call on the error path: returns {true,true} on any internal
 // failure or when the inputs preclude meaningful diagnosis (empty model,
@@ -17,7 +18,7 @@ func (s *OpenAIGatewayService) DiagnoseModelAvailabilityForPlatform(
 	ctx context.Context,
 	groupID *int64,
 	requestedModel string,
-	_ string,
+	platform string,
 ) ModelAvailabilityDiagnosis {
 	if s == nil {
 		return ModelAvailabilityDiagnosis{HasAccountsInPool: true, HasModelSupport: true}
@@ -27,7 +28,7 @@ func (s *OpenAIGatewayService) DiagnoseModelAvailabilityForPlatform(
 		return ModelAvailabilityDiagnosis{HasAccountsInPool: true, HasModelSupport: true}
 	}
 
-	accounts, err := s.listSchedulableAccounts(ctx, groupID)
+	accounts, err := s.listSchedulableAccounts(ctx, groupID, platform)
 	if err != nil {
 		// Conservative fallback so the caller keeps returning 503; we do not
 		// want a transient lookup failure to flip into 404 model_not_found.