fix: cancel tools and clean up child processes

[wangtsiao]

Summary

• propagate cancellation to running exec and MCP tool calls
• clean up stdio process trees on Unix and Windows, including abrupt owner exit
• avoid the Windows MSVC LIBCMT linker warning for the bundled code-search MCP binary
• add regression coverage and documentation

Validation

• cargo fmt --all -- --check
• cargo test -p devo-code-search-mcp
• local build validation passed