perf(dpi/quic): drop redundant dcid.clone() in long-header parser

[obchain]

Summary

parse_long_header_packet_with_length was allocating the DCID bytes twice per packet:

let dcid = payload[offset..offset + dcid_len].to_vec();
quic_info.connection_id = dcid.clone();
// ...
extract_tls_from_long_header_packet(packet_data, &mut quic_info, &dcid, ...);

The intermediate dcid Vec only existed to feed extract_tls_* a &[u8], and that function already accepts a borrowed slice — no reason to materialize a second owned copy when the bytes are sitting in payload.

This change captures the DCID byte-range, assigns quic_info.connection_id directly from payload[dcid_range], and reborrows the same slice for the extract_tls_* call. The borrow checker is fine with this — payload: &[u8] and the &mut QuicInfo reference disjoint memory.

The short-header path had the same shape and was fixed in #317; the long-header path was missed. This brings the two paths into parity.

Closes #342.

Behavior

No change. quic_info.connection_id ends up holding the same bytes; extract_tls_* receives the same DCID slice it did before. Verified by:

cargo test --lib network::dpi::quic   # 11 passed
cargo test --lib                       # 371 passed
cargo fmt --check                      # clean
cargo clippy --all-targets -- -D warnings  # clean

Test plan

• All QUIC DPI unit tests pass
• Full cargo test --lib
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings