fix(files_sharing): skip unresolvable share recipients

[Antreesy]

• Resolves: [Bug] Talk fails to upload files to the conversation, if one of participants is gone (orphan attendee from former LDAP user) spreed#18415

Summary

in SharesUpdatedListener, a share recipient may reference a user id that no backend can resolve anymore (e.g. with LazyUser::getUID()) - like remnant / incorrectly removed user.

Example: Talk application supplies LazyUsers from oc_talk_attendees table, which can get desynced from the user backend (e.g. LDAP entries)

Skip this recipient instead of aborting the share operation resolve the issue. As listener is running as a post-effect, and share is already created (at least to the room -> to the group), it should be a safe reason to catch and skip?

To test:

1. Easiest path: create a rogue entry in oc_talk_attendees, with non-existing id, omit capability 'conversation-subfolders' to utilize legacy share mechanism (each file is separate share), share a file to the room => POST request should fail with 500
2. Complicated path: find a way for LDAP to clear mapping / execute similar flow to leave a rogur entry, then same as in opt.1

TODO

• Sanity check
• Possible side-effects?
• For backports - should use OC\User\NoUserException ?

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

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