fix: redirect stdio MCP stderr to logs

[he-yufeng]

Summary

• route stdio MCP subprocess stderr to ~/.kimi/logs/mcp/<server>.log instead of the interactive terminal
• keep non-stdio MCP server setup on the existing MCPConfig path
• add a focused regression test for sanitized per-server log paths

Notes

This addresses the stderr leak described in #2251. It is separate from MCP logging notifications handled in #1637; those are in-protocol log messages, while this patch covers the child process stderr stream that FastMCP 3.2.4 defaults to sys.stderr.

To verify

• uv run pytest tests\core\test_mcp_stdio_logging.py -q -p no:cacheprovider --basetemp .tmp\pytest
• uv run pytest tests\cli\test_mcp_oauth.py::test_load_mcp_tools_treats_unreadable_oauth_storage_as_unauthorized -q -p no:cacheprovider --basetemp .tmp\pytest-oauth
• uv run ruff check src\kimi_cli\soul\toolset.py tests\core\test_mcp_stdio_logging.py
• uv run ruff format --check src\kimi_cli\soul\toolset.py tests\core\test_mcp_stdio_logging.py
• uv run pyright src\kimi_cli\soul\toolset.py tests\core\test_mcp_stdio_logging.py

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve transforming stdio MCP configs

_build_mcp_client now treats any StdioMCPServer instance as a plain stdio transport and constructs StdioTransport directly, which bypasses server_config.to_transport(). In FastMCP, transforming stdio configs (e.g., with tools/include_tags/exclude_tags) subclass StdioMCPServer, so this branch silently drops those transforms and exposes unfiltered tools whenever users configure a transforming stdio server. The previous Client(MCPConfig(...)) path preserved that behavior, so this is a functional regression for transformed stdio MCP setups.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[he-yufeng]

Addressed the review point. Plain StdioMCPServer configs still get a StdioTransport with a log file, but transforming stdio configs now keep the FastMCP MCPConfig transport path so tools / include_tags / exclude_tags are preserved.

Validated locally:

• .venv\Scripts\python.exe -m pytest tests\core\test_mcp_stdio_logging.py -q --basetemp .tmp\pytest -p no:cacheprovider
• .venv\Scripts\python.exe -m ruff check src\kimi_cli\soul\toolset.py tests\core\test_mcp_stdio_logging.py
• .venv\Scripts\python.exe -m ruff format --check src\kimi_cli\soul\toolset.py tests\core\test_mcp_stdio_logging.py
• .venv\Scripts\python.exe -m py_compile src\kimi_cli\soul\toolset.py tests\core\test_mcp_stdio_logging.py
• git diff --check