test: add unit tests for RPC framing, SDK state machine, and server domain services
Add comprehensive tests across three layers:
- RPC framing: empty payloads, max boundary, truncated frames, multi-frame buffers,
all status codes, all method ID ranges, payload-too-large for response/push
- SDK: event broadcast send/receive, multiple subscribers, clone preservation,
conversation upsert, missing conversation, message ID roundtrip, member keys
- Server domain: auth session validation/expiry, channel creation/symmetry/validation,
delivery peek/ack/sequence ordering/fetch-limited, key package upload/fetch/validation,
hybrid key batch fetch, size boundary tests
- CI: MSRV (1.75) check job, macOS cross-platform build check
Christian Nennemann
872695e5f1