feat(elt-common): Make datetime watermarks possible

[WHTaylor]

ref #321

The electricity_sharepoint pipeline needs a datetime watermark, and it seems likely we'll want this for future pipelines as well.

Summary by CodeRabbit

• New Features

  • Watermarks now support datetime values.
  • Datetimes are serialised using ISO 8601 format and restored automatically when reading watermark data.

• Bug Fixes

  • Improved validation and error messaging for unsupported watermark values.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 99037c14-d2e1-415b-91c8-f22b769ad1ca

📥 Commits

Reviewing files that changed from the base of the PR and between 882b265 and 01829bc.

📒 Files selected for processing (2)

• elt-common/src/elt_common/extract.py
• elt-common/tests/unit_tests/test_extract.py

---

📝 Walkthrough

Walkthrough

Changes

Watermark datetime support

Layer / File(s)	Summary
Watermark datetime codec elt-common/src/elt_common/extract.py	Watermark.value accepts datetimes, serialises them as ISO-8601 strings, and deserialises ISO-8601 strings back to datetime objects with updated validation.
Datetime validation coverage elt-common/tests/unit_tests/test_extract.py	Tests centralise invalid-value messages and cover datetime deserialisation.

Possibly related PRs

• ISISNeutronMuon/analytics-data-platform#347: Adds the preceding Watermark JSON serialisation, deserialisation, and scalar-value validation.

Suggested reviewers: martyngigg

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately highlights the main change: adding datetime watermark support in elt-common.
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.

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[martyngigg]

Good to go from my perspective.