Build: Add .venv/ to .gitignore (#3626)

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# Rationale for this change

`.venv/` is the default directory name created by `python -m venv .venv`
and is the convention recommended by the Python docs and modern tooling
like `uv`. Currently `.gitignore` only ignores `venv/` but not `.venv/`,
so contributors using the dotted convention could accidentally stage
their virtualenv.

## Are these changes tested?

N/A — this is a `.gitignore`-only change.

## Are there any user-facing changes?

No.

Author: qzyu999

.gitignore

@@ -23,6 +23,7 @@ __pycache__/
 .tox/
 env/
 venv/
+.venv/
 *.egg-info/
 test-reports
 build/