Check the user ID in the embedded device keys

[poljar]

Adds a missing check for the user ID in the embedded device keys.

• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.92%. Comparing base (777ce05) to head (b8bc629).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-crypto/src/olm/account.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6553      +/-   ##
==========================================
- Coverage   89.92%   89.92%   -0.01%     
==========================================
  Files         381      381              
  Lines      106860   106865       +5     
  Branches   106860   106865       +5     
==========================================
+ Hits        96095    96098       +3     
- Misses       7108     7114       +6     
+ Partials     3657     3653       -4     

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

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing poljar/better-device-keys-check (b8bc629) with main (777ce05)}