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/server/collectives.py

@@ -236,7 +236,7 @@ def configure_collectives_tools(mcp: FastMCP):
 
     @mcp.tool(
         title="Update Collective",
-        annotations=ToolAnnotations(idempotentHint=False, openWorldHint=True),
+        annotations=ToolAnnotations(idempotentHint=True, openWorldHint=True),
     )
     @require_scopes("collectives:write")
     @instrument_tool