Avoid shell=True when launching mcp dev inspector on Windows

[raashish1601]

Summary

• resolve npx to a concrete executable on Windows instead of probing with shell=True
• keep mcp dev launching the inspector with a direct argv call and copied environment
• add focused CLI tests for the Windows resolution path and the shell-free inspector launch

Fixes #1257.

Validation

• PYTHONPATH=src python -m pytest tests/cli/test_utils.py -q
• python -m ruff check src/mcp/cli/cli.py tests/cli/test_utils.py
• git diff --check

[maxisbey]

Closing in favor of #2524, which removes the now-stale # pragma: no cover along with the same shutil.which() change. Thanks for the contribution.

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