Use sys.executable instead of tee for the stdio placeholder command - #4478

Summary

tests/mcp/helpers.py resolved tee at import time and asserted on the result:

tee = shutil.which("tee") or ""
assert tee, "tee not found"

tee is a Unix utility. It is not on PATH on a default Windows install, so the assert fires during collection and takes down every module that imports this helper. That is 11 modules, seven under tests/mcp/ plus tests/voice/test_pipeline.py, tests/test_agent_as_tool.py, tests/test_process_model_response.py and tests/test_tool_origin.py.

The value is only ever used as MCPServerStdio(params={"command": tee}), and those tests patch mcp.client.stdio.stdio_client with DummyStreamsContextManager, so the command is never executed. It only has to be a path that exists. sys.executable satisfies that on every platform by definition.

CI does not see this because GitHub's windows-latest image ships Git for Windows with its usr/bin on PATH, which provides tee.EXE. A Windows machine without that directory on PATH does not have it.

Test plan

Same checkout, same machine, only PATH differing, on Windows 11 / Python 3.13.11:

without C:\Program Files\Git\usr\bin on PATH, before this change
  $ uv run pytest tests/mcp tests/voice/test_pipeline.py tests/test_agent_as_tool.py \
      tests/test_process_model_response.py tests/test_tool_origin.py
  Interrupted: 10 errors during collection
  ERROR tests/mcp/test_caching.py - AssertionError: tee not found

with that directory on PATH (what a GitHub runner has), before this change
  shutil.which("tee") -> C:\Program Files\Git\usr\bin\tee.EXE
  6 passed

without it on PATH, after this change
  622 passed, 24 skipped in 89.40s

uv run ruff format --check and uv run ruff check are clean on the changed file.

make typecheck reports three errors on this machine, all in src/agents/sandbox/, none touched here. They are platform-conditional rather than caused by this change: mypy src gives 3 errors and mypy --platform linux src gives Success: no issues found on the same checkout. The typecheck job runs on ubuntu-latest, so CI does not see them either. Filed separately as #4477 rather than folded in here.

Issue number

n/a

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR

The verification script is a shell script and this was verified on Windows, so I ran the underlying commands directly rather than through it. No new tests: this restores collection of existing ones, and the pass count above is the check.