fix: dynamically resolve standalone Python patch version

[bigcat88]

The python-build-standalone project releases every 1-2 weeks, and each release only ships one patch version per minor series (e.g. 3.12.13 replaces 3.12.12). Since we had the full patch version hardcoded in constants.py, every new release broke standalone downloads with 404 errors until someone bumped the constant and cut a new comfy-cli release. This already happened three times (3.12.8 -> 3.12.12 -> 3.12.13).

This change pins only the minor version ("3.12") in the constant and resolves the exact patch version at download time by fetching the release's SHA256SUMS file (~45 KB) and parsing it. Full X.Y.Z versions still work if passed explicitly.

Also updates the GitHub URLs from indygreg/python-build-standalone to astral-sh/python-build-standalone (the project moved).

Follows up on #366.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #369      +/-   ##
==========================================
+ Coverage   52.52%   53.14%   +0.62%     
==========================================
  Files          32       32              
  Lines        3568     3588      +20     
==========================================
+ Hits         1874     1907      +33     
+ Misses       1694     1681      -13     

Files with missing lines	Coverage Δ
comfy_cli/constants.py	100.00% <100.00%> (ø)
comfy_cli/standalone.py	59.61% <100.00%> (+25.09%)	⬆️

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