Tests: Document value comparisons in get_adjacent_post tests

[huzaifaalmesbah]

Description

Reviewed tests/phpunit/tests/link/ for ticket #64895.

There are 29 calls to assertEquals() in tests/phpunit/tests/link/getAdjacentPost.php:

• test_get_adjacent_post(): 13 calls comparing WP_Post objects or arrays of WP_Post objects returned from get_adjacent_post().
• test_get_adjacent_post_exclude_self_term(): 8 calls comparing WP_Post objects returned from get_adjacent_post().
• test_get_adjacent_post_caching(): 8 calls comparing WP_Post objects returned from get_adjacent_post().

All 29 calls intentionally compare WP_Post instances by value:

1. get_adjacent_post() queries the database and instantiates fresh WP_Post objects, which are distinct instances from the fixture references in the test.
2. assertSame() strictly checks object identity (===), which fails when comparing distinct WP_Post instances representing the same post.

As noted specifically by committer @lancewillett in Trac #64895 (comment:93):

"link/getAdjacentPost.php, the largest single file, compares WP_Post objects, where assertEquals() is the right call. For those, a short note in the test saying the loose comparison is deliberate counts as done."

These intentional loose comparisons are preserved and documented with:
// Keep assertEquals() because the objects are intentionally compared by value.

Testing Instructions

npm run test:php -- --test-suffix .php tests/phpunit/tests/link/getAdjacentPost.php
composer run lint tests/phpunit/tests/link/getAdjacentPost.php

Trac ticket: https://core.trac.wordpress.org/ticket/64895

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity
Model(s): Gemini
Used for: Inspecting assertions and drafting documentation comments. Implementation and test verification were reviewed and executed locally.

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[lancewillett]

PR #13935 landed in https://core.trac.wordpress.org/changeset/64067