Add column field log to task pipeline

[pooleycodes]

What type of PR is this?

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Description

Extends TaskPipeline to detect and surface missing mandatory fields from a
column-field log as task rows.

A new column_field_path parameter (alongside mandatory_fields) can be passed
to TaskPipeline.run(). When provided, the pipeline reads the column-field CSV,
compares the field column against the supplied mandatory fields list, and emits
one column-field task row per missing field — using the same schema and SHA-256
reference hash as the existing log and issue sources.

Key behaviours:

• mandatory_fields entries can be a plain string (field must be present) or a
  list of strings (any one of the alternatives is sufficient)
• If column_field_path is given without mandatory_fields, a warning is logged
  and the check is skipped gracefully
• If the path does not exist, a warning is logged and it is skipped
• Fixes _transform_issues_to_tasks to guard against None organisation/endpoint
  (was passing None directly to pl.lit)
• run() now consistently returns self.status rather than the bare enum value

Related Tickets & Documents

• Ticket Link Create task pipeline in DL-Python #526

QA Instructions

Pass a column-field CSV (columns: dataset, resource, column, field) and a
mandatory_fields list to TaskPipeline().run(). Rows for any fields absent from
the CSV will appear in the output with task-source=column-field and details
containing {"field": "...", "issue_type": "missing-field"}.

Added/updated tests?

• Yes — TestColumnFieldTasks in tests/unit/pipeline/test_task.py

[codecov-commenter]

Codecov Report

❌ Patch coverage is 15.62500% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.40%. Comparing base (c8265f6) to head (91a9366).
⚠️ Report is 51 commits behind head on main.

Files with missing lines	Patch %	Lines
digital_land/pipeline/task.py	15.62%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #554      +/-   ##
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- Coverage   86.54%   86.40%   -0.15%     
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  Files          92       93       +1     
  Lines        6466     6641     +175     
==========================================
+ Hits         5596     5738     +142     
- Misses        870      903      +33     

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[gibahjoe]

LGTM