optimise batch report

[rhaegar325]

Improve JSON readability of batch report failures

Background

In the moppy_batch_report.json produced by batch_processing, each entry in the failures section is hard to read: both error_message and stderr_tail hold multi-line text. Once JSON-escaped (newlines become \n), they collapse into a single very long line.

Changes

Multi-line fields in failure entries are now emitted as arrays of lines, so that with json.dumps(indent=2) each line renders on its own row.

• Add _as_lines() helper: empty → None, single line → plain string, multi-line → list of lines.
• Change _stderr_tail return type to str | list[str] | None (string for a single line, list for multiple lines); no longer uses "\n".join.
• Apply _as_lines to error_message in both tasks and failures, keeping the field type consistent.
• Bump SCHEMA_VERSION from v1 to v2 (field shape changed).

Before / After

Before (one very long line):

"stderr_tail": "line1\nline2\nline3\n...(20 lines)...",
"error_message": "job 1 | exit_status=1 | err_tail:\nTraceback\n  boom"

After (one line per entry):

"stderr_tail": [
  "line1",
  "line2",
  "line3"
],
"error_message": [
  "job 1 | exit_status=1 | err_tail:",
  "Traceback",
  "  boom"
]

Short messages (e.g. "input missing") stay plain strings.

Compatibility

• The dashboard reads directly from the database and does not parse this JSON, so the field type change does not affect other functionality.
• Existing report files are not updated automatically; regenerate to get the new format.

Testing

• Updated stderr_tail and schema-version assertions.
• Added _as_lines behavior test and a multi-line error_message splitting test.
• tests/unit/test_batch_report.py: 25 passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.4%. Comparing base (ed7102d) to head (61c9189).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #475   +/-   ##
=====================================
  Coverage   78.4%   78.4%           
=====================================
  Files         33      33           
  Lines       6483    6491    +8     
  Branches    1226    1228    +2     
=====================================
+ Hits        5082    5090    +8     
  Misses      1128    1128           
  Partials     273     273           

Flag	Coverage Δ
unit	78.4% <100.0%> (+<0.1%)	⬆️

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