fix: thread remaining buffer through sync_read responses

[jimsynz]

Summary

• At high baud rates (1Mbaud), multiple servo responses arrive faster than individual UART reads can consume them
• The previous implementation discarded extra bytes after parsing each response, causing subsequent reads to fail or return corrupted data
• Thread the remaining buffer from each read_sync_response into the next call
• Propagate remaining bytes through receive_response for consistency

Test plan

• All existing tests pass (mix check --no-retry)
• Tested on hardware with 6 STS3215 servos at 1Mbaud — sync_read now returns correct values for all servos consistently