dtls13: reject malformed ClientHello extensions

[zRedShift]

Summary

• Reject DTLS 1.3 ClientHello extension vectors that do not consume the remaining ClientHello body.
• Preserve ClientHellos with no extension vector.
• Add regressions for zero-length and underdeclared extension vectors with trailing bytes.

Validation

• cargo fmt --check
• git diff --check
• cargo test --all-targets --features rcgen
• cargo clippy --all-targets --features rcgen -- -D warnings
• cargo test --no-default-features --features rust-crypto
• cargo clippy --no-default-features --features rust-crypto -- -D warnings
• cargo test --doc --features rcgen