fix: guard against None version in _get_version_notes

[rtibblesbot]

Plan: Fix TypeError: '<=' not supported between instances of 'int' and 'NoneType' in _get_version_notes when channel_version.version is None by adding a None guard that returns all version notes in that case.

• Add regression test for None channel_version.version
• Fix the None guard in _get_version_notes

Summary

Fixes Sentry issue STUDIO-J6Z — a TypeError when ChannelVersion.version is None prevents the channel lookup API from returning results. When version is None, including all version notes is the correct behaviour (there's no version ceiling to filter against).

References

Closes #5832

Reviewer guidance

Run pytest contentcuration/kolibri_public/tests/test_public_v1_api.py -v to verify no regressions. The new test test_channel_version_with_none_version_returns_all_version_notes exercises the None-version path directly.

AI usage

Implemented with Claude Code following a pre-approved plan using TDD. The fix and regression test were reviewed for correctness and the full API test suite was run before committing.

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• Rebased onto the target branch and resolved any conflicts
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