Add fuzz tests for pgproto3 Decode methods and fix discovered panics

Add 23 fuzz targets covering all complex Decode methods plus
Frontend.Receive and Backend.Receive. Fuzzing uncovered 5 additional
bugs:

- FunctionCall.Decode: no bounds checks before reading fixed fields,
  panics on short input
- Query.Decode: empty input causes slice bounds panic (src[:-1])
- CopyFail.Decode: same empty input panic as Query
- SASLInitialResponse.Decode: skips 4 bytes without checking they exist
- NegotiateProtocolVersion.Decode: untrusted uint32 optionCount used as
  slice capacity, enabling OOM via malicious message

Co-Authored-By: Claude Opus 4.6 <[email protected]>

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