feat(auth): add support for managing custom headers in auth files

Closes #2457

internal/watcher/synthesizer/file.go

@@ -157,6 +157,7 @@ func synthesizeFileAuths(ctx *SynthesisContext, fullPath string, data []byte) []
 			}
 		}
 	}
+	coreauth.ApplyCustomHeadersFromMetadata(a)
 	ApplyAuthExcludedModelsMeta(a, cfg, perAccountExcluded, "oauth")
 	// For codex auth files, extract plan_type from the JWT id_token.
 	if provider == "codex" {
@@ -233,6 +234,11 @@ func SynthesizeGeminiVirtualAuths(primary *coreauth.Auth, metadata map[string]an
 		if noteVal, hasNote := primary.Attributes["note"]; hasNote && noteVal != "" {
 			attrs["note"] = noteVal
 		}
+		for k, v := range primary.Attributes {
+			if strings.HasPrefix(k, "header:") && strings.TrimSpace(v) != "" {
+				attrs[k] = v
+			}
+		}
 		metadataCopy := map[string]any{
 			"email":             email,
 			"project_id":        projectID,