fix: address PR review feedback (round 6)

- Fix assign_tag sending Content-Type header with no body
- Mark collectives_update_collective as idempotent (no ETag involved)
- Raise OCSError when 'data' key missing instead of silent fallback
- Tighten color validator to 3 or 6 hex chars only
- Add comment explaining null emoji semantics in set_page_emoji

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

nextcloud_mcp_server/models/collectives.py

@@ -60,7 +60,7 @@ class CollectiveTag(BaseModel):
     @field_validator("color")
     @classmethod
     def validate_hex_color(cls, v: str) -> str:
-        if not re.fullmatch(r"[0-9A-Fa-f]{3,8}", v):
+        if not re.fullmatch(r"[0-9A-Fa-f]{3}(?:[0-9A-Fa-f]{3})?", v):
             raise ValueError(f"Invalid hex color: {v!r}")
         return v