[TASK] Address type issues in FileSearchQuery

The $result property's doc type is causing issues
for PHPStan. Switching it to a typed property and
state resolves them.

Other types have been filled in as well as they're
private so safe.

Used command:

> ./Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #98002
Releases: main, 11.5
Change-Id: Ic960858eee4cd40ed010762a44dec944f5114e4a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75263


Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>