refactor: Make ClientBuilder safer to use when restoring a session

[jmartinesp]

Changes:

• Add some docs to discourage using the ClientBuilder methods that modify the server or homeserver URLs when restoring an existing session.

• Removes the unused ffi::Session::homeserver_url field, since it can lead to clients believing that modifying that value may have an effect in the SDK behaviour.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.61%. Comparing base (9ab886f) to head (b8f5642).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5874      +/-   ##
==========================================
+ Coverage   88.59%   88.61%   +0.01%     
==========================================
  Files         363      363              
  Lines      102888   102888              
  Branches   102888   102888              
==========================================
+ Hits        91153    91171      +18     
+ Misses       7496     7475      -21     
- Partials     4239     4242       +3     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5874 will not alter performance

_{Comparing refactor/safer-client-builder (b8f5642) with main (5d396e4)}

Summary

✅ 50 untouched

[andybalaam]

Looks like we need a complement-crypto update to go with this?

https://github.com/matrix-org/matrix-rust-sdk/actions/runs/19459574501/job/55680392867?pr=5874

Error: internal/api/rust/rust.go:119:4: unknown field HomeserverUrl in struct literal of type matrix_sdk_ffi.Session

[jmartinesp]

Looks like we need a complement-crypto update to go with this?

matrix-org/matrix-rust-sdk/actions/runs/19459574501/job/55680392867?pr=5874

Error: internal/api/rust/rust.go:119:4: unknown field HomeserverUrl in struct literal of type matrix_sdk_ffi.Session

Yes, I didn't realise this would affect complement-crypto.

[jmartinesp]

Added CC PR here and retrying the bindings test.

[jmartinesp]

@andybalaam it's working now!