feat(oauth): expose sessions expiration errors during QR login

[Johennes]

MSC4108 designates 404 / M_NOT_FOUND to non-existing (and possibly expired) rendezvous sessions. This introduces dedicated NotFound errors so that consumers of the SDK can handle this case accordingly.

• Public API changes documented in changelogs (optional)

[codspeed-hq]

CodSpeed Performance Report

Merging #5898 will not alter performance

_{Comparing Johennes:johannes/not-found (13b2273) with main (45a9d96)}

Summary

✅ 50 untouched

[codecov]

Codecov Report

❌ Patch coverage is 93.60000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.66%. Comparing base (ec44c74) to head (13b2273).
⚠️ Report is 35 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...atrix-sdk/src/authentication/oauth/qrcode/grant.rs	92.12%	7 Missing and 3 partials ⚠️
.../matrix-sdk/src/authentication/oauth/qrcode/mod.rs	81.25%	3 Missing ⚠️
.../src/authentication/oauth/qrcode/secure_channel.rs	90.62%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5898      +/-   ##
==========================================
+ Coverage   88.65%   88.66%   +0.01%     
==========================================
  Files         362      362              
  Lines      103146   103363     +217     
  Branches   103146   103363     +217     
==========================================
+ Hits        91440    91645     +205     
- Misses       7458     7471      +13     
+ Partials     4248     4247       -1     

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[poljar]

Makes sense, thanks for the extensive tests.