Fix: Remove user memoization

[Koc]

User memoization in the Activity app causes incorrect activity notifications in edge cases, particularly when activities are generated about files changed by other users. This was observed in the Forms app.

Reproducer:

1. Alice creates a form with submissions linked to file1.xlsx
2. Bob creates a form with submissions linked to file2.xlsx
3. Both forms are submitted quickly
4. Wait for SyncSubmissionsWithLinkedFileJob to execute

Root cause:
The memoized user state persists across console commands and background jobs, even when the user context changes via:

$user = $this->userManager->get($ownerId);
$this->userSession->setUser($user);

Solution:
Remove user memoization at the Activity level since it's not needed and causes state leakage between different user contexts.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[Koc]

Psalm failures looks a bit unrelated... All other pipelines are green

[artonge]

Some were related @Koc :

Error: lib/CurrentUser.php:63:11: RedundantCast: Redundant cast to string (see https://psalm.dev/262)