Warn about corrupted reads in DIAMOND output parser

[wwood]

Motivation

• Make the error raised when parsing DIAMOND output more actionable because DIAMOND can emit malformed rows when input reads are corrupted.
• Help users quickly diagnose cases where a ValueError: not enough values to unpack originates from bad FASTQ/FASTA input rather than DIAMOND itself.

Description

• Update the exception in singlem/diamond_spkg_searcher.py to include guidance recommending validation of the input FASTQ/FASTA files when an unexpected DIAMOND output line is encountered.
• Add a focused unit test test/test_diamond_spkg_searcher.py that mocks Popen to emit a malformed DIAMOND row and verifies the new guidance is included in the raised exception.

Testing

• Ran pixi run -e dev pytest test/test_diamond_spkg_searcher.py and the new test passed.
• Ran the full suite with pixi run -e dev pytest test which completed successfully with 224 passed, 25 skipped, 1 warning.
• Running python -m pytest test/test_diamond_spkg_searcher.py outside the pixi dev environment failed due to the missing extern dependency and is not representative of CI results.

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Codex Task