chore(backend): drop pre-Reader metric-value code

[lewisjared]

Removes the dead metric-value helpers left behind by the results-reader adoption in #39.

• Trims core/metric_values.py to MetricValueType and parse_id_list, the only symbols the routes still use.
• Deletes core/outliers.py and the AnnotatedScalarValue model. Outlier detection now happens inside the reader via OutlierPolicy.
• Drops the mock-based tests that covered the removed helpers. The MetricValueCollection builder tests still cover the live reader path.

Net -1147 lines with no behaviour change. Direct tests for core/reader_values.py are planned as a follow-up.

Summary by CodeRabbit

• Removed Features

  • Removed automatic outlier detection and related outlier annotations from metric values.
  • Metric results now use the standard scalar and series value representations.

• Bug Fixes

  • Updated invalid comma-separated ID handling to avoid returning a specialised HTTP 400 response.

• Chores

  • Removed unused metric-value helpers and their associated test coverage.

[netlify]

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[coderabbitai]

📝 Walkthrough

Walkthrough

Removed legacy outlier detection and annotated scalar-value helpers, deleted their tests, changed invalid ID parsing error propagation, and updated the changelog.

Changes

Legacy metric-value cleanup

Layer / File(s)	Summary
Remove outlier and annotated value flow backend/src/ref_backend/core/outliers.py, backend/src/ref_backend/models.py, backend/tests/test_core/test_outliers.py, backend/tests/test_core/test_metric_values.py	The outlier utilities, AnnotatedScalarValue, and related outlier, CSV, and scalar-processing tests were removed.
Update metric-value parsing backend/src/ref_backend/core/metric_values.py, changelog/42.trivial.md	Imports were reduced, invalid ID parsing errors are no longer converted to HTTP 400 responses, and the changelog records the helper removal.

Possibly related PRs

• Climate-REF/ref-app#39: Uses the reader-based metric-value flow associated with the removed outlier and annotated-value machinery.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: removing obsolete pre-Reader metric-value code from the backend.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/delete-dead-metric-value-code

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