refactor: remove length-prefixed logic for PQ key shares

[FreezB11]

Problem

s2n-tls has supported multiple draft revisions of the PQ hybrid TLS 1.3
key share format while PQ standards were still in flux. Early drafts
required each sub-share (ECC + PQ) inside the combined KeyShareEntry to
carry its own 2-byte length prefix. This was only ever needed for draft
standards; s2n-tls has committed to supporting drafts (tls13_pq_hybrid_draft_revision < 5)
only through the end of 2025.

Draft revision 5 — the format used for ML-KEM support — dropped the
inner length prefixing entirely, since share sizes are fixed per named
group and the outer TLS 1.3 KeyShareEntry length is already sufficient
to parse the blob.

Solution

This PR removes the len_prefixed field from s2n_kem_params and all
logic that branches on it, since s2n-tls now only needs to support
draft revision 5 going forward.

Source changes:

• tls/s2n_kem.h — remove the len_prefixed field
• tls/s2n_kem.c — collapse the length-prefix branches in
  s2n_kem_send_public_key, s2n_kem_recv_public_key,
  s2n_kem_send_ciphertext, and s2n_kem_recv_ciphertext down to the
  unprefixed (revision 5) behavior
• tls/extensions/s2n_key_share.[ch] — drop the len_prefixed
  parameter from s2n_ecdhe_send_public_key
• tls/extensions/s2n_client_key_share.c /
  tls/extensions/s2n_server_key_share.c — remove the logic that set
  or inferred len_prefixed for TLS 1.3 negotiation
• tls/s2n_kem_preferences.[ch] — remove
  s2n_tls13_client_must_use_hybrid_kem_length_prefix and related
  plumbing

Out of scope / explicitly preserved:

• TLS 1.2 PQ key exchange (tls/s2n_client_key_exchange.c,
  tls/s2n_server_key_exchange.c) is unconditionally length-prefixed
  and is a separate wire format from the TLS 1.3 draft cleanup this
  issue targets. That behavior is preserved as-is and not touched by
  this PR.

Test changes:

• tests/unit/s2n_server_key_share_extension_test.c — remove the
  len_prefixed loop, test only the unprefixed path
• tests/unit/s2n_client_key_share_extension_pq_test.c — same, plus
  update s2n_copy_pq_share and s2n_generate_pq_hybrid_key_share_for_test
  helpers to drop the prefix parameter
• tests/unit/s2n_kem_test.c — remove length-prefixed assertions from
  send/recv public key and ciphertext tests
• tests/unit/s2n_tls13_pq_handshake_test.c — remove
  len_prefix_expected from test vectors and handshake assertions
• tests/unit/s2n_pq_mlkem_policies_test.c — replace the
  length-prefix check with an assertion that ML-KEM policies use draft
  revision 5

Testing

• s2n_pq_mlkem_policies_test: 725 assertions pass
• s2n_server_key_share_extension_test: 353 assertions pass
• s2n_client_key_share_extension_pq_test: 39 assertions pass
• s2n_kem_test: 167 assertions pass
• s2n_tls13_pq_handshake_test: compiles and passes (pre-existing,
  unrelated cert-path failure not introduced by this change)
• grep -rn "len_prefixed" tls/ tests/ returns no results

Resolves #5606

[alexw91]

Thanks for this! Can you also remove .tls13_pq_hybrid_draft_revision everywhere too? That variable should only ever contain 0 or 5, and was the value that would be branched on to decide whether to length prefix or not. Now that we're no longer supporting length prefixing, we don't need that variable either.

[FreezB11]

@alexw91 sure i can

[FreezB11]

thanks for running the actions the issue earlier was the aws-lc i recreated it on my machine and then fixed it, now will go ahead with removing tls13_pq_hybrid_draft_revision

[FreezB11]

@alexw91 @CarolYeh910 would love some review.