Reject unoffered cipher suite in TLS 1.3 client

[spwalgren]

The client only checked that the selected cipher was a TLS 1.3 cipher at the negotiated version, so a server could pick any TLS 1.3 cipher even when the client offered a restricted subset.

Also added UnsupportedCipherSuite-TLS13 runner test.

Context and motivation

The client only checked that the selected cipher was a TLS 1.3 cipher at the negotiated version, so a server could pick any TLS 1.3 cipher even when the client offered a restricted subset.

Description of changes

The client's cipher-suite validation now additionally requires that the selected cipher appear in the client's configured cipher list rather than checking protocol version alone.

Something to note: The default client configuration offers all supported TLS 1.3 ciphers so those handshakes are unaffected. Clients that configure their own ciphers can have handshakes fail.

Testing

Added UnsupportedCipherSuite-TLS13 to the runner suite and existing tls tests still pass.

Review considerations

Compatibility: Handshakes that previously succeeded will now fail when a server selects a TLS 1.3 cipher outside the client's configured list. This is the specified behavior and affects only non-conforming peers, but it is a behavior change for clients that both restrict their suites and talk to such a peer.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[github-actions]

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Please review before merging.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.20%. Comparing base (f3798b6) to head (0b1e900).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3537      +/-   ##
==========================================
- Coverage   78.41%   78.20%   -0.21%     
==========================================
  Files         700      700              
  Lines      125889   125886       -3     
  Branches    17413    17410       -3     
==========================================
- Hits        98711    98451     -260     
- Misses      26305    26563     +258     
+ Partials      873      872       -1     

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