[2.x] fix(mentions): prevent deleted fallback for unsynced posts during formatting

[huoxin233]

Fixes #4823

Changes proposed in this pull request:

This PR modifies all Format*Mentions and Unparse*Mentions classes to:

• Use the relation's loaded collection if it has been eager loaded (via relationLoaded), executing 0 queries.
• Fallback directly to the base model query (e.g., Post::query()->find()) rather than querying the pivot table if the relation isn't loaded.

This completely bypasses the pivot table requirement during text evaluation, allowing the formatters to accurately retrieve display names for edited posts.

This bug also caused false positives in the Flarum test suite:

• editing_a_post_with_deleted_author_that_has_a_mention_works: The test edited a post by sending the exact same text that the post already had. Because the broken unparse() method previously evaluated the old post content as [deleted], Flarum thought the content had changed and dispatched the Revised event. With unparse() fixed, Flarum correctly recognizes the text did not change and skips the Revised event, causing the test's pivot table assertion to fail. This PR updates the test payloads to actually mutate the content so that the Revised event fires properly.
• deleted_tag_mentions_relation_unparse_and_render_as_expected: The test verified that a post mentioning a Tag would render as [deleted] if the pivot row was missing, despite the Tag actually existing in the database fixtures. Before our format fixes, Flarum wrongly rendered this as deleted because it strictly trusted the empty pivot table. With our format logic fixed, Flarum rightfully discovers the Tag still exists and refuses to render it as deleted, causing the test to fail. This PR updates the test to target a genuinely non-existent Tag ID (999) to accurately test the deleted fallback.

Output of $post->content

Content use to test:

@"huoxin"#p89 abc
@"huoxin"#1 abc

Output:

Before:
[2026-07-22T18:44:19.913160+00:00] flarum.INFO: [Test] Content observed during Saving event: {"content":"@\"[unknown]\"#p89 abc
@\"[deleted]\"#1 abc"} 

After:
[2026-07-22T18:42:36.145018+00:00] flarum.INFO: [Test] Content observed during Saving event: {"content":"@\"huoxin\"#p89 abc
@\"huoxin\"#1 abc"} 

Also tested with email notification which relates to #3454, but was not able to reproduce the issue, the username is showing correctly before and after the fix:

Notification

Hey huoxin,

huoxin233 mentioned you in a post in cccddaaas.

https://xxxxx

---

@"huoxin"#p89 abc
@"huoxin"#1 abc

- The test team -
....

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)