feat(protocol): expose dynamic MCP health fields

[wylangyue]

Summary:

• extend dynamic_mcp context items with status, tool_count, and last_error_code
• keep the new fields optional for rolling compatibility with older AgentDock nodes
• retain strict rejection of undeclared dynamic MCP context fields

This is the protocol prerequisite for AgentDock to distinguish configured integrations from healthy, discoverable dynamic MCP capabilities without exposing raw error messages.

Tests:

• go test ./...

Consumer implementation:

• feat(mcp): report dynamic server health in context agentdock#68 adds the runtime fields and will update the protocol dependency after this schema change is released.