Fix/chunked frostd message - Permitting larger transactions by adding chunking (copied from #195)

[scaraven]

test: Added missing test coverage for signing package encryption at both the cipher and coordinator comms level. Small transactions like payment-zkapp serialize, encrypt, and fit within frostds 65535-byte message limit. Large transactions like deploy-v0.0.4 with verification keys produce a serialized SigningPackage of 92KB which exceeds both the Noise protocol frame limit and frostds message size limit because frost-core hex-encodes the message bytes roughly doubling the payload size.

test: Added chunked encrypt-decrypt roundtrip test proving that splitting the large deploy-v0.0.4 signing package into chunks, encrypting each chunk separately, decrypting and reassembling on the other side produces the original payload with each chunk fitting within frostds MAX_MSG_SIZE limit. This demonstrates the strategy for the fix.

fix: Large signing packages (e.g. deploy transactions with verification keys) exceeded frostds 65535-byte message limit after JSON serialization hex-encoded the transaction bytes from 46KB to 92KB. The coordinator now chunks the serialized payload so each encrypted chunk fits in a single frostd message, sending a 4-byte chunk count header first followed by N encrypted chunks per recipient. The participant collects the expected number of chunks across polling cycles, decrypts each in order, and reassembles the original payload before deserializing. Cipher is unchanged, chunking lives entirely in the coordinator and participant HTTP comms layer.

Acknowledgements

Original pr by @jk89

[jCabala]

The chunks code seems a bit messy right now. Especially in participant. I think we should move this to a separate function. Also confusedd abotu the gathering logic. Why do we repeat it after loop and within loop.