Adds dot_rejects_non_dot_alpn to assert the rustls server enforces
ALPN strictness rather than silently accepting a mismatched
negotiation. This is the load-bearing behavior behind the cross-
protocol confusion defense — without enforcement, the ALPN "dot"
advertisement is just a sign hung on an unlocked door.
Refactors test_tls_configs to return the leaf cert DER instead of a
prebuilt client config, and adds a dot_client(cert_der, alpn) helper
so each test can build a client config with the ALPN list it needs.
The five existing DoT tests gain one line each to call dot_client
with dot_alpn(); behavior unchanged.
127/127 tests pass.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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