test/fuzz: xrow_(decode_error, header_decode, decode_call, decode_dml) corpus updated

Summary

• add corpus for xrow_decode_error and xrow_header_decode
• update corpus for xrow_decode_call and xrow_decode_dml for increased coverage
• coverage report is now added as an artifact of fuzz-coverage job

xrow.c coverage is now 28.19% which should satisfy the coverage criteria.

MR is best viewed by commits, as the commit which introduces new corpus is rather big.

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• Closes https://git.picodata.io/picodata/picodata/picodata/-/issues/541
• Closes https://git.picodata.io/picodata/picodata/picodata/-/issues/465
• Docs follow-up: not necessary