add frame decoder recovery tests and fix header size mismatch

[bilalabsh]

Summary
Closes #2

The frame codec had basic encode/decode tests, but nothing covered malformed wire input or whether the decoder recovers cleanly after a bad frame. The header was also wrong — encoder wrote 20 bytes of header while FRAME_HEADER_SIZE is 24, so round-trip decode was already broken on main.

This PR fixes that header mismatch, adds reserved-byte validation and an accepted_frames counter on FrameDecoder, and adds deterministic regression tests for truncated input, integrity failures, and post-failure recovery. No live services involved.

What Changed
backend/src/protocol/codec.rs
Fixes header encoding/decoding: reserved field is 8 bytes (was 4), matching FRAME_HEADER_SIZE = 24.
Rejects non-zero reserved bytes with ProtocolError::InvalidMessage and drains the bad frame from the buffer.
Adds accepted_frames counter and accepted_frames() getter on FrameDecoder, incremented only on successful decode.
backend/src/protocol/frame_tests.rs
New test module with helpers for building valid/malformed frame fixtures.

8 tests covering:

Truncated header → Ok(None), no panic
Truncated payload → Ok(None), no panic
Oversize payload length → MessageTooLarge
Non-zero reserved bytes → InvalidMessage
Checksum corruption → ChecksumMismatch
Failed decode leaves accepted_frames at 0 and buffer in a sane state
Valid frame decodes successfully after checksum failure
Valid frame decodes successfully after invalid reserved bytes
backend/src/protocol/mod.rs
Wires in frame_tests under #[cfg(test)].
diagnostic/
Ran python3 build.py on this branch.

How I Tested
cd backend
cargo test --lib protocol::frame_tests
cargo test --lib protocol::codec::tests
cargo test --lib
8/8 new frame tests passing
4/4 existing codec tests passing
24/24 full lib suite passing