Fix CI failures on OTP 27+/28 caused by undersized test keys

[mtrudel]

Summary

• test/hap/hap_session_transport_test.exs failed on every OTP 27.x/28.x CI job (all Elixir versions), while every OTP 26.x job passed — see run https://github.com/mtrudel/hap/actions/runs/30280957121.
• Failure: ** (ErlangError) Erlang error: {:badarg, {"aead.c", 90}, "Unknown cipher or invalid key size"} from :crypto.crypto_one_time_aead(:chacha20_poly1305, ...).
• Root cause: the tests used 1-byte stand-in keys (<<1>>, <<2>>) instead of the 32-byte keys chacha20_poly1305 actually requires. OTP 26's :crypto NIF tolerated the undersized key; OTP 27+ (linked against a newer OpenSSL) validates AEAD key length strictly and raises badarg.
• This is unrelated to any recent app changes — it's a pre-existing latent test bug that would fail identically on main under OTP 27/28.

Fix

Replace the 1-byte keys with 32-byte keys (:binary.copy(<<1>>, 32) / :binary.copy(<<2>>, 32)), including in the raw :crypto.crypto_one_time_aead/6 call used to independently verify the wire format in the second test.

Test plan

• mix test test/hap/hap_session_transport_test.exs passes locally (OTP 29, which also enforces strict key-size validation)
• mix test passes in full (41 tests, 0 failures)

🤖 Generated with Claude Code