fix(vertex): include prefix in auth filename and validate at import

Address two blocking issues from PR review:
- Auth file now named vertex-{prefix}-{project}.json so importing the
  same project with different prefixes no longer overwrites credentials
- Prefix containing "/" is rejected at import time instead of being
  silently ignored at runtime
- Add prefix to in-memory metadata map for consistency

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/auth/vertex/vertex_credentials.go

@@ -31,7 +31,8 @@ type VertexCredentialStorage struct {
 	// Type is the provider identifier stored alongside credentials. Always "vertex".
 	Type string `json:"type"`
 
-	// Prefix optionally namespaces models for this credential (e.g., "teamA/gemini-2.0-flash").
+	// Prefix optionally namespaces models for this credential (e.g., "teamA").
+	// This results in model names like "teamA/gemini-2.0-flash".
 	Prefix string `json:"prefix,omitempty"`
 }