fix(mcp): honor env PATHEXT for stdio commands

[ooiuuii]

Fixes #56536

What does this PR do?

• Resolves stdio MCP commands against the same environment that will be passed to the MCP subprocess.
• Honors Windows PATHEXT from the MCP env when a config supplies both PATH and PATHEXT.
• Preserves Windows-style Path / Pathext casing instead of creating a duplicate PATH key while prepending the resolved command directory.
• Adds focused regression coverage for env-scoped PATHEXT and mixed-case Windows env keys.

Why?

_resolve_stdio_command() already supports resolving commands against a filtered MCP PATH, but on Windows shutil.which(..., path=...) still consults the parent process PATHEXT. A stdio MCP config with its own PATHEXT could therefore fail to resolve a command that is valid in the configured subprocess environment.

Proof

• python -m py_compile tools\\mcp_tool.py tests\\tools\\test_mcp_tool_issue_948.py
• Focused Windows proof script passed:

process PATHEXT = .EXE
MCP env PATH = <temp>\\bin
MCP env PATHEXT = .CMD
<temp>\\bin\\demo.cmd exists
_resolve_stdio_command(demo, env) resolves demo.cmd
focused MCP PATHEXT proof passed

Limitations

• python -m pytest tests\\tools\\test_mcp_tool_issue_948.py -q could not run in this checkout because pytest is not installed (No module named pytest).
• Repo-local/global autoreview helper was not available in this checkout.

[teknium1]

Thanks for addressing the Windows MCP environment-resolution mismatch. Current main still calls shutil.which() from tools/mcp_tool.py:587-589 after _build_safe_env() merges the configured subprocess environment at tools/mcp_tool.py:426-446, so the fix targets a live path.

Problems

• tests/tools/test_mcp_tool_issue_948.py:87 creates demo.cmd, but the same new test supplies PATHEXT=.CMD. The proposed resolver derives demo.CMD; since the test only patches sys.platform and runs on Linux's case-sensitive filesystem, its os.path.isfile() lookup cannot find the lowercase fixture.

Suggested changes

• Make the fixture extension match the configured PATHEXT casing (for example demo.CMD), or mock the filesystem check while retaining a platform-independent regression test.

Automated hermes-sweeper review.

[teknium1]

This fixture is demo.cmd, while the test configures PATHEXT=.CMD. _which_in_env() constructs demo.CMD; with only sys.platform patched, Linux CI uses a case-sensitive filesystem and will not find this fixture. Please match the fixture casing or mock the lookup.