Fix: Preserve version ordering in VersionResponse

[drawliin]

Closes #857

What this changes

This updates packagedb.package_managers.VersionAPI.get_until() to preserve the original version order returned by fetch().

Previously, VersionResponse.valid_versions and newer_versions were built using sets. That removed duplicates, but it also lost the original ordering of versions.

This change switches the returned values to ordered lists and keeps deduplication with internal seen sets.

Why

There is already a related note in packagedb/package_managers.py:

# FIXME: DO NOT use set() for storing version lists: they lose the original ordering

Preserving version order makes the behavior more predictable while still avoiding duplicate version values.

Tests

Updated packagedb/tests/test_package_managers.py to:

• expect ordered results from get_until()
• add a regression test that checks both order preservation and deduplication