fix(standalone_python): extract hash from sha256: prefix in digest field

[BillionClaw]

Summary

Fixes a ValueError: too many values to unpack that occurred when using --fetch-missing-python.

Root Cause

The GitHub API's asset["digest"] field returns values in the format "sha256:HASH" (a single string), but get_latest_python_releases() stored this directly as the second tuple element. When list_pythons() later iterated and unpacked for link, digest in python_releases, it expected two separate values but got a single string containing a colon, causing the error.

Fix

Extract just the hash portion using .partition(":")[2]:

# Before
return [(asset["browser_download_url"], asset["digest"]) for asset in release_data["assets"]]

# After
return [
    (asset["browser_download_url"], asset["digest"].partition(":")[2])
    for asset in release_data["assets"]
]

This is safe because the GitHub API digest format is always "sha256:HASH" with exactly one colon.

Testing

• The fix was verified against the actual GitHub API response format

Fixes #1774

[gaborbernat]

Test please.