Honor explicit cookie cleanup after automatic collection

[OskarEichler]

Summary

Run explicitly requested cleanup even after the automatic collection counter resets, and pass MozillaStore cleanup through to its in-memory session store. Automatic cleanup frequency on add remains unchanged.

Reproduction and verification

With gc_threshold: 1, adding a session cookie runs automatic cleanup and resets the counter; subsequent cleanup(true) does not remove it. MozillaStore additionally never forwards cleanup to its session jar. Both stores now pass this reproduction plus 12 assertions covering thresholds 1/100, persistent-cookie retention, ordinary cleanup and repeated session cleanup.

Verified this patch independently on master with Ruby 4.0.6 and SQLite3 2.9.6: existing rake test suite 144 tests, 2,870 assertions, zero failures/errors. Targeted syntax and git diff --check pass. Comparative RuboCop Lint adds no offenses (baseline 40; cleanup patch removes one). This repository does not configure a Ruby lint suite, so the comparison is supplementary, not a claim of clean full lint.

Focused checks are external scratch scripts. No repository tests were added or modified because this contribution's task explicitly prohibits test-file changes. All data is synthetic; SQLite checks use temporary/in-memory local databases. Other Ruby versions and upstream CI are not locally verified. Runtime source at installed v1.1.6 is identical apart from its version constant; consumer backports will retain that release instead of adopting the unreleased version/Ruby requirement.

Compatibility / breaking changes

No API changes. Explicit cleanup now performs its requested work even without new insertions. MozillaStore cleanup(true) removes in-memory session cookies. Explicit cleanup can therefore perform a scan where it previously incorrectly returned early; automatic add-triggered cadence is unchanged.