fix: log warning when native MCP config returns no tools

[Aboudjem]

Summary

• Initialize tools_list = [] before the async event-loop block in _resolve_native() to prevent UnboundLocalError when asyncio.run() raises a RuntimeError that falls through without assignment
• Add logger.warning() after tool listing + filtering in _resolve_native(), matching the existing pattern in _resolve_external(), so users get a clear diagnostic when their native MCP server (Stdio/HTTP/SSE) returns no tools or when tool_filter removes all of them
• Add 3 tests covering: empty server response, filter removing all tools, and the UnboundLocalError scenario

Closes #5116

Test plan

• TestResolveNativeNoToolsWarning::test_logs_warning_when_server_returns_no_tools — verifies warning is logged when MCP server returns empty tool list
• TestResolveNativeNoToolsWarning::test_logs_warning_when_tool_filter_removes_all_tools — verifies warning is logged when tool_filter rejects every tool
• TestResolveNativeNoToolsWarning::test_no_unbound_local_error_on_failed_async_run — verifies no UnboundLocalError when asyncio.run() raises an unmatched RuntimeError

🤖 Generated with Claude Code

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Note

Low Risk
Low risk: only adjusts native MCP tool discovery to handle empty results/edge-case errors more gracefully and adds tests; no protocol or execution-path changes when tools are present.

Overview
Native MCP tool resolution (_resolve_native) now initializes tools_list defensively and logs a warning + returns empty when the server returns no tools (or when tool_filter removes them all), aligning behavior with external MCP URL resolution.

Adds tests to verify the warning is emitted for empty discovery, filter-all scenarios, and to prevent UnboundLocalError when async discovery fails.

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