fix(12): resolve critical code review findings (CR-01 through CR-04)

- CR-01: fix browse URL from /folders/{id} to /items?parent_ref= (broken feature)
- CR-02: remove raw provider exception text from _TerminalProviderError messages
- CR-03: add user_id predicate to get_or_create_folder_state query (ownership boundary)
- CR-04: add max_length=255 to ConnectionRenameRequest.display_name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/api/cloud/schemas.py

@@ -9,7 +9,7 @@ from __future__ import annotations
 from datetime import datetime
 from typing import List, Optional
 
-from pydantic import BaseModel, field_validator
+from pydantic import BaseModel, Field, field_validator
 
 
 # ── Capability / item schemas ─────────────────────────────────────────────────
@@ -75,7 +75,7 @@ class ConnectionRenameRequest(BaseModel):
     Only display_name is accepted — mass assignment prevention.
     """
 
-    display_name: str
+    display_name: str = Field(..., max_length=255)
 
     @field_validator("display_name")
     @classmethod