#1529: match complete cache age directive names

[gemshrine]

Fixes #1529.

The cache-age scan and replacement matched max-age and s-maxage anywhere in the joined header. An x-max-age or x-s-maxage extension before the actual freshness directive was rewritten instead, leaving the real age below the configured minimum.

Require the directive to start at the beginning of the header or after a comma separator, preserving that separator and the directive case during replacement. The regression covers both directive names in lowercase and mixed/uppercase forms, checking that extensions remain unchanged.

Checks: 33 SQLite store tests pass (124 assertions). The new regression fails on unchanged master. RuboCop and git diff --check pass. Selected runs disable the whole-project coverage threshold without changing CI configuration.

The full test suite was also run: 601 tests, 1631 assertions, 2 failures, 1 errors, 9 skips. Its failures are test_check_all_bills, test_fake_pull_request_base_repo_matches_requested_repo and the timing-sensitive test_processes_fact_when_slot_fits_the_timeout. The same three failures were reproduced on unchanged master f888443 with test ordering seed 58188; the full suite is not green.