Expose HumanQrGrantLoginError::Unknown reason in error message

[hughns]

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

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

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing hughns:hughns/HumanQrGrantLoginError-unknown-reason (568a6a9) with main (6777907)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.90%. Comparing base (6777907) to head (568a6a9).
⚠️ Report is 8 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6514      +/-   ##
==========================================
- Coverage   89.93%   89.90%   -0.03%     
==========================================
  Files         381      381              
  Lines      105781   105781              
  Branches   105781   105781              
==========================================
- Hits        95132    95101      -31     
- Misses       7007     7029      +22     
- Partials     3642     3651       +9     

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