fix(tests): resolve Python 3.14 Windows CI failures caused by subprocess DLL issues

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Summary

Two independent Windows-only failures on Python 3.14 caused by subprocess pipe handles incompatible with the MinGW64 git runtime and uv-managed venv DLL isolation.

Changes

Root cause 1 — _ctypes DLL load failure in test_main.py (2 FAILED)

sys.executable in a uv temp venv can't load _ctypes in child processes because os.add_dll_directory() entries are not inherited across process boundaries. Replaced subprocess-spawning tests with in-process equivalents:

• test_main_entry_point → typer.testing.CliRunner
• test_main_direct_execution → runpy.run_path() (matches existing test_main_if_name_main_block pattern)

Root cause 2 — STATUS_DLL_INIT_FAILED (0xC0000142) from MinGW64 git (86 ERRORS)

shutil.which("git") resolves to C:\Program Files\Git\mingw64\bin\git.EXE on this runner. That binary fails DLL init when spawned with capture_output=True (PIPE handles) on Python 3.14 Windows. Production code uses bare "git" (which resolves to the Windows-native cmd\git.exe wrapper) and has no such issue.

Changes across conftest.py, test_e2e_bump_release.py, test_bump_command.py:

• Removed GIT = shutil.which("git") or "git" — use bare "git" to match production code
• Changed capture_output=True → stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL for all fixture git calls that discard output (belt-and-suspenders; calls that read stdout retain capture_output=True)

Testing

• make test passes locally
• make fmt has been run
• New tests added (or explain why not needed) — existing 493 tests now all pass; no new tests required

Checklist

• Commit messages follow the Conventional Commits format
• CHANGELOG.md entry added (or not needed for this change)
• Documentation updated if behaviour changed
• make deptry passes (no unused or missing dependencies)

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