feat(common): add the Thread variant to OwnedLinkedChunkId and LinkedChunkId

[bnjbvr]

This should make it clearer what's the expected behavior for the IDB implementation of the event cache. Also, testing is good, because this was actually incorrectly implemented in the memory store.

I've added it in the IDB implementation as well, because I think it should fail (and can't run the IDB tests locally for some reason). [edit] Yes, it did fail, so I've removed it from this PR; someone can investigate it later.

Part of #5122, so part of #4869 too.

[codecov]

Codecov Report

❌ Patch coverage is 98.52941% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.89%. Comparing base (ca89700) to head (59c63ab).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-common/src/linked_chunk/mod.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #5445    +/-   ##
========================================
  Coverage   88.89%   88.89%            
========================================
  Files         333      333            
  Lines       91884    92009   +125     
  Branches    91884    92009   +125     
========================================
+ Hits        81677    81791   +114     
- Misses       6376     6387    +11     
  Partials     3831     3831            

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[poljar]

Looks good, let us just add an changelog entry.