Tests: Restore Ajax hooks for each test

[sirreal]

Summary

• Restores the Ajax callback baseline during every test setup so a preceding test cannot permanently remove callbacks required by another case.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 100 consecutive fresh randomized Ajax-group runs passed on the final combined campaign head; each completed 190 tests and 1,212 assertions with one expected skip.
• Exact seeds 1789050001 through 1789050100 all completed within the 30-second hard bound.
• The final gate also covered the non-querying locale restoration carried by the l10n PR; the Ajax callback change itself had already passed the original 50-run isolated gate.
• Replayed the original failing seed and focused contaminator/consumer cases during diagnosis.
• PHP syntax checks passed for every changed PHP file.
• git diff --check passed.

Trac: https://core.trac.wordpress.org/ticket/65893

[lancewillett]

Refreshed with current trunk in a non-rewriting merge: ee9071a.

Validation passed: git diff --check; vendor/bin/phpcs --standard=phpcs.xml.dist for the changed Ajax fixture; and a focused randomized Ajax replay (seed 13191): 15 tests, 31 assertions.

The prior one-off CI timeout had no assertion failure. Fresh CI has started and will provide matrix coverage; local validation was targeted.

[github-actions]

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[github-actions]

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[lancewillett]

Landed in https://core.trac.wordpress.org/changeset/63593

[lancewillett]

Landed in https://core.trac.wordpress.org/changeset/63593