[TASK] Use proper PSR-4 compatible namespaces 1

This change replaces invalid namespaces in some files to
ensure PSR-4 loading compatibility.

Additional missing namespace autoloading configuration for
`tstemplate` test files is added to root composer.json.

Resolves: #97791
Releases: main
Change-Id: I52a473f71dd284a3ecb7e0bda397331db3b4b47d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74931


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>