fix(hooks): use the Windows virtualenv interpreter

[audmartina9]

On Windows, the pre-commit hook creates .venv/Scripts/python.exe but then tries to install dependencies using .venv/bin/python, which raises FileNotFoundError before validation starts. Select the Windows interpreter path on os.name == "nt" and keep the existing POSIX path elsewhere.

Adds one offline integration test that exercises the hook's dependency branch using a real virtual environment, an empty requirements file, and three fixture validators. Each validator asserts that it runs inside a virtual environment. Downloads and index checkout are replaced with local fixtures; the interpreter and pip calls are real.

Validation on Windows:

• Reproduced the original FileNotFoundError at upstream commit 168495987009158d811c7bb41a3bb4b65d9a7a80 using the same offline fixture approach.
• python -m unittest discover -s git-hooks -p test_pre_commit_venv.py -v — passed (1 test).
• git diff --check — passed before commit.
• This is a tooling change; no CSV values or schemas are modified. The offline test does not run the production dataset validation pipeline. Linux/macOS execution has not been tested here.

Prepared and tested with an AI coding assistant under the account holder's authorization. If this tooling fix is eligible for a discretionary USDC/USDT contributor reward, please assess it under the project's rules. I understand that the published per-cell data reward and schema-DBIP reward do not automatically cover this repair; no payment has been agreed or is being claimed as owed.

If this tooling contribution qualifies for a reward, Ethereum mainnet receiving address: 0xb45A8D75cDf5C85c15d046D6ceA826Db4c034C61.