Continuing from #20244

Infrastructure changes

• replaced uses to strncpy() with either strncpyz() or memcpy() depending on whether the length is known.
• removed obsolete RRDLABELS * from parser->user. Replaced it with a counter.
• fixed RRDLABELS cleanup on errors, to avoid leaking memory.
• universal USER_AUTH structure for web_clients.

Websocket

• fixed deadlock while shutting down
• added URL parameters for controlling max frame size per connection
• USER_AUTH is copied from web clients

API Changes

• added scope_instances, scope_labels, scope_dimensions to /api/v3/data and /api/v3/weights.
• added options instances, dimensions, labels to /api/v3/contexts, allowing the caller to get full visibility of all metadata per context.
• added cardinality_limit to all contexts (instances, dimensions, labels)
• added cardinality_limit to /api/v3/data
• added cardinality_limit to /api/v3/weights for the MCP format
• re-implemented full text search on contexts for provide all the information detail required for the caller to understand what is matched - respects cardinality limit
• added option rfc3339 to /api/v3/data and nodes and contexts metadata responses
• added option long-keys to /api/v3/data to respond with meaningful json keys
• added contexts registry to maintain the count of unique contexts across all nodes
  • added to /api/v3/info
• added option rfc3339 to /api/v3/alerts
• added option long-keys to /api/v3/alerts
• updated swagger

MCP

• metadata tools (list_metrics, get_metrics_details, list_nodes, get_node_details)
• working query metrics tool (query_metrics)
• working functions (list_functions, execute_function)
• working weights (find_correlated_metrics, find_anomalous_metrics, find_unstable_metrics)
• working logs (systemd-journal, windows-events)
• removed resources implementation
• implemented stdio bridges for converting stdio <-> websockets (for use with Claude Desktop and Claude Code)
• implemented MCP ping (responses only, Netdata does not request pings)
• implemented MCP batch requests
• mockup of all 2025 MCP spec endpoints
• each MCP client is now associated with a USER_AUTH.