Packetgen: Fix potential runtime panics in rust generated code

- Replace unwrapping of Enum, Struct, CustomField, Array field
  conversion, raise Error instead
- Fix bounds checks for vector fields with _size_ or _count_
  length
- Fix bounds checks for payload fields

BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines
Bug: 225384813
Test: reproduction with minimal PDL source

Change-Id: If5c2c5b7fe01fe602ed93ccf1bfdfc7c86c12770