[2.x] fix(admin): correct tree array construction in GeneralSearchSource

[imorland]

Array.prototype.push() returns the new length (a number), not the array. Using it as the tree argument caused result.tree.map() to throw "e.tree.map is not a function" when rendering admin search results for any setting with a tree property.

Replace .map(extractText).push(label) with [...map(extractText), label] to return an array as expected.

**Fixes #4372 **

Changes proposed in this pull request:

Reviewers should focus on:

Screenshot

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)