fix(ranges): require matching pre-release policy in difference

[notatallshaw]

intersection and union raise on operands with different configured pre-release policies; difference did not, so a - b succeeded where the equivalent a & ~b raises, and dropped a pre-release the subtrahend's policy excludes (e.g. >=1.0 minus >=2.0b1 with prereleases=False drops 2.0b1, which is in a and not in b).

[henryiii]

Codex likes to check ===. I don't think this is something to worry about for this PR, but might be useful for later.

🤖 AI text below 🤖

The new set-relation API can report incorrect subset/superset results for ranges with arbitrary-equality literal admissions. This is a user-visible semantic bug in the added functionality.

Review comment:

• [P2] Account for literal admissions in subset checks — /Users/henryfs/git/pypa/packaging/src/packaging/ranges.py:810-810
  When self has an === literal that other does not, this fallback can return True incorrectly because other.complement() is one-way for literals and does not admit every non-matching literal. For example, (SpecifierSet('===a').to_range() | SpecifierSet('===b').to_range()).is_subset(SpecifierSet('===a').to_range()) returns True even though 'b' is contained only in the left side; is_superset() inherits the same false positive.

[henryiii]

Does keeping only self's pre-release policy matter if you now throw an error if they don't match? (in the description)

[notatallshaw]

Yes, the error is thrown on the configured policy being different (prerelease=None, True, False), not on the implied policy (prerelease=None and <1 vs. <1.0dev0).