feat(qr-code): Add continuation sender to WaitingForAuth

[hughns]

This PR adds a continuation sender to the WaitingForAuth enums so that applications which suspend or navigate away while the verification URI is open (e.g. on Android) can resume the process explicitly.

This is based on @poljar's work in #6173

• I've documented the public API changes in the appropriate changelog files (see Writing changelog entries).
• This PR was made with the help of AI.

Signed-off-by: Hugh Nimmo-Smith hughns@element.io

[codecov]

Codecov Report

❌ Patch coverage is 77.58621% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.92%. Comparing base (805f981) to head (19ac9e6).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...atrix-sdk/src/authentication/oauth/qrcode/grant.rs	79.06%	9 Missing ⚠️
.../matrix-sdk/src/authentication/oauth/qrcode/mod.rs	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6711      +/-   ##
==========================================
- Coverage   89.94%   89.92%   -0.02%     
==========================================
  Files         397      397              
  Lines      110751   110793      +42     
  Branches   110751   110793      +42     
==========================================
+ Hits        99611    99636      +25     
- Misses       7367     7382      +15     
- Partials     3773     3775       +2     

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[codspeed-hq]

Merging this PR will improve performance by 35.23%

⚡ 1 improved benchmark
✅ 49 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	Restore session [memory store]	226.9 ms	167.8 ms	+35.23%

Tip

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_{Comparing hughns:hughns/qr-waiting-for-auth-continuation (19ac9e6) with main (805f981)}