Fix path-resolution bug in changenotes.mts validate by using getChangenotes

[mario-campos]

isValidChangenoteFile receives a file name (no leading path) and then attempts to read that file, regardless of the CWD. If the user's CWD is not in unreleased-change-notes, then the command will likely fail to open the file.

Incidentally, for #4155, I've added getChangenotes, which consolidates some duplicative code between the assemble and validate commands. We can take advantage of that here in validate, and in so doing, fix the bug.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Workflow types:

• N/A

Products:

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Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

How will you know if something goes wrong after this change is released?

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Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.