ci: install astral managed Python so prerelease setup is cached

[bdraco]

The big chunk of time in the test matrix is actions/setup-python downloading and installing Python from manifest. On Windows + 3.14t (free threaded) that step alone takes around 50 seconds because the prerelease is not pre-baked into the hosted-tools cache.

Move the test and benchmark jobs to astral managed Python through a small composite action mirroring the home-assistant/core#171760 approach: pin uv (no manifest fetch on cache miss), request the interpreter via setup-uv with cache-python: true, then eagerly uv python install so cached venvs that point at the managed python resolve their symlinks on restore. Poetry is pointed at the managed interpreter with poetry env use $(uv python find $version) on cache miss.

Venv cache key bumped from v2 to v3 to discard older entries that pointed at the actions/setup-python interpreter.

Lint and build_wheels left alone; lint is already fast and cibuildwheel manages its own pythons.

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks

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_{Comparing ci/astral-managed-python (dfbb4d3) with main (bb76920)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.05%. Comparing base (78504ba) to head (dfbb4d3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   95.94%   96.05%   +0.11%     
==========================================
  Files          14       14              
  Lines        1922     1976      +54     
  Branches      232      247      +15     
==========================================
+ Hits         1844     1898      +54     
  Misses         42       42              
  Partials       36       36              

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