Fix TCP message handling panics and oversized allocations

[Aditya30ag]

Summary

This PR improves robustness of TCP message handling by preventing panics and unsafe allocations when receiving malformed input.

Changes

• Replace TCP receive expect()/panic paths with proper error propagation.
• Add maximum message size checks before allocating buffers.
• Reject oversized length prefixes from untrusted peers.
• Add regression tests for:
  • Truncated TCP messages causing incomplete reads.
  • Oversized message length prefixes.

Previously, malformed TCP input could:

• Trigger a panic when the peer disconnected before sending the full message.
• Cause excessive memory allocation attempts by sending a very large length prefix.

These changes ensure invalid input results in an error instead of crashing the process.

Testing

Tested with:

cargo test -p ll_mp test_recv_tcp_msg
cargo clippy -p ll_mp --tests

#3888 

[domenukk]

if you want this to be 100% correct you also should check for size before adding 4 to the len (since at this point it could already have overflown for MAX_INT)

[Aditya30ag]

Good catch. I changed the length calculation to use checked_add(4) before applying the maximum message size check, so the addition is now protected against integer overflow.