feat: structured MCP error codes and failure diagnostics (closes #6352)

[vakrahul]

feat: Structured MCP error codes & failure diagnostics (closes #6352)

Summary

Implements structured error handling across MCP registry operations so
users can quickly self-diagnose issues instead of seeing raw Python
tracebacks.

Changes

New: core/framework/runner/mcp_errors.py

• MCPErrorCode enum with all 7 required error codes:
  MCP_INSTALL_FAILED, MCP_AUTH_MISSING, MCP_CONNECT_TIMEOUT,
  MCP_TOOL_NOT_FOUND, MCP_PROTOCOL_MISMATCH,
  MCP_VERSION_CONFLICT, MCP_HEALTH_FAILED
• MCPError base class enforcing What/why/fix message format
• Subclasses: MCPToolNotFoundError, MCPAuthError,
  MCPInstallError, MCPConnectTimeoutError,
  MCPProtocolMismatchError, MCPVersionConflictError,
  MCPHealthCheckError

Modified: core/framework/runner/mcp_client.py

• Replaced raw ValueError in call_tool with MCPToolNotFoundError
• Added [Server: X] [Transport: Y] context to all tool call
  error paths in _call_tool_stdio_async and _call_tool_http

Modified: core/framework/runner/tool_registry.py

• register_mcp_server now emits structured log on both success
  and failure: {server, status, tools_loaded, skipped_reason}

New: core/tests/test_mcp_errors.py

• 5 unit tests — all passing on Ubuntu and Windows CI

Requirements Covered

ID	Requirement	Status
DX-1	Structured error codes	✅
DX-2	What/why/fix error messages	✅
DX-4	Structured startup logging	✅
DX-5	Server context in tool call errors	✅

Note

mcp_registry.py does not yet exist in the codebase — it is being
created in #6349 by @Antiarin.