refactor(dpi/stun): drop unreachable!() arm in class decode

[0xghost42]

Summary

Same shape as #279 for SNMP: collapse a match whose catch-all is provably unreachable into one that no longer needs it.

analyze_stun decodes the message class from two bits:

let c0 = (msg_type >> 4) & 0x1;
let c1 = (msg_type >> 8) & 0x1;
let class_bits = (c1 << 1) | c0;

Each of c0 / c1 is masked to a single bit, so class_bits is bounded to 0..=3 by construction. The match below already enumerated all four values and carried a _ => unreachable!() arm that the compiler could not prove away.

This PR:

• Tightens c0 / c1 from u16 to u8, keeping the bit arithmetic narrow.
• Replaces the unreachable!() with the 0b11 case (ErrorResponse), with a short comment recording the bound that justifies the change.
• Adds test_class_bits_exhaustive_for_unknown_method that pairs every class with a non-Binding method (0x0003) so a future change to method handling can't silently make class decode method-dependent — the four existing per-class tests all use method=Binding and would still pass.

No behavior change in the success path; the _ arm previously panicked and now returns ErrorResponse, which is what unreachable!() was implicitly claiming would never be needed.

Verification

• cargo test --lib: 360 passed, 0 failed (15 in network::dpi::stun, incl. the new test).
• cargo clippy --all-targets -- -D warnings: clean.
• cargo fmt --check: clean.

Notes

Reviewed RFC 5389 §6 (message type encoding) to confirm the four class encodings and the meaning of the M / C bits.

[domcyrus]

@0xghost42 thanks a lot! This also looks good to me!