Cache API: Avoid redundant cache invalidation in wp_insert_* functions during bulk meta input

[MicahelE]

Summary

Fixes #65485.

When inserting posts, comments, or users with multiple meta entries via meta_input or comment_meta, the wp_cache_set_*_last_changed() callback (hooked to added_*_meta/updated_*_meta) fired once per meta key, causing N redundant cache writes for N meta entries.

For example, inserting a post with 5 meta_input entries triggered 5 redundant last_changed cache writes during the meta loop — even though wp_insert_post() already calls clean_post_cache() at the end, which invalidates the cache once.

The fix

Temporarily suspends the per-key _last_changed callback during the bulk meta loop and restores it afterward. The parent functions (clean_post_cache(), etc.) already invalidate the cache once at the end of the insertion, so no explicit re-trigger is needed.

Affected functions

• wp_insert_post() — meta_input loop
• wp_insert_comment() and wp_update_comment() — comment_meta loops
• wp_insert_user() — meta loops (both add and update branches)

Testing

Reproduction (before fix)

POST: posts last_changed writes = 5 (for 5 meta_input entries)
COMMENT: comment last_changed writes = 5 (for 5 comment_meta entries)
USER: users last_changed writes = 18 (for 5 meta_input entries + role meta)

After fix

POST: posts last_changed writes = 3 (clean_post_cache + 2 single _pingme/_encloseme adds after loop)
COMMENT: comment last_changed writes = 1
USER: users last_changed writes = 6 (role-setting + contact-method meta outside loop)

The redundant per-key invalidations during the bulk loops are eliminated. Remaining writes are from legitimate single meta operations outside the loop.

Test suite

• New test: Tests_Cache_RedundantMetaInvalidation (5 tests) — passes with fix, fails without it
• Existing tests pass with 0 regressions:
  • Post/Meta: 527 tests
  • Comment: 547 tests
  • User: 1341 tests
  • Cache: 92 tests
• PHP lint and WordPress coding standards: clean

How to test

npm run test:php -- --filter Tests_Cache_RedundantMetaInvalidation

Trac ticket: https://core.trac.wordpress.org/ticket/65485

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