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

[huoxin233]

Fixes #4823 & #3454
Backport fix from #4824 to 1.x

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"#p34 abc
@"huoxin"#1 abc

Output:

Before:
[2026-07-22 19:05:01] flarum.INFO: [Test] Content observed during Saving event: {"content":"@\"[unknown]\"#p34 abc
@\"[deleted]\"#1 abc"} 

After:
[2026-07-22 19:07:39] flarum.INFO: [Test] Content observed during Saving event: {"content":"@\"huoxin\"#p34 abc
@\"huoxin\"#1 abc"} 

This seems to fix issue related to email notification #3454 also:

Before:

[2026-07-22 19:05:01] flarum.INFO: Message-ID: <95f9f5663f4c9b91ec6f56f6588ee9c4@0.0.0.0>
Date: Wed, 22 Jul 2026 19:05:01 +0000
Subject: [Follow User] New post in asdsa
From: test <admin@xxxxxx>
To: huoxin <a@a.com>
MIME-Version: 1.0
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable
.....
    <div class="content">
        <p>Hey huoxin,</p>

<p>huoxin233 posted in a discussion: asdsa</p>

<p>To view the new activity, check out the following link:</p>

<p><a href="https://xxxxxxxx/d/9/9">https://xxxxxxxx/d/9/9</a></p>

<p>—</p>

<p>@“[unknown]”#p34 abc
@“[deleted]”#1 abc</p>

<p>—</p>

<p>You won’t receive any more notifications about this discussion until you’re up-to-date.</p>


    </div>
    <div class="footer">
        <div class="content">
            <p>Sent from test using FoF Pretty Mail</p>
        </div>
    </div>
    </body>
</html>

After:

[2026-07-22 19:07:39] flarum.INFO: Message-ID: <c976d05041b1ec05578ddf134a5da263@0.0.0.0>
Date: Wed, 22 Jul 2026 19:07:39 +0000
Subject: [Follow User] New post in asdsa
From: test <admin@xxxx>
To: huoxin <a@a.com>
MIME-Version: 1.0
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

....
    <div class="content">
        <p>Hey huoxin,</p>

<p>huoxin233 posted in a discussion: asdsa</p>

<p>To view the new activity, check out the following link:</p>

<p><a href="https://xxxxxxxx/d/9/10">https://xxxxxxxx/d/9/10</a></p>

<p>—</p>

<p>@“huoxin”#p34 abc
@“huoxin”#1 abc</p>

<p>—</p>

<p>You won’t receive any more notifications about this discussion until you’re up-to-date.</p>


    </div>
    <div class="footer">
        <div class="content">
            <p>Sent from test using FoF Pretty Mail</p>
        </div>
    </div>
    </body>
</html>

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)