Separate "audit" workflow

[fbacall]

Summary of changes

• Move gem auditing into separate workflow which only runs if Gemfile/Gemfile.lock was modified.
• Also run it automatically on Monday @ 9AM - but possibly this is unnecessary with dependabot... remove if it is just flagging duplicate issues.

Motivation and context

It was annoying having the test workflow "fail" due to no fault of the committed code.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.

(Squash before merge)

[Copilot]

Pull request overview

This PR separates Ruby gem vulnerability auditing from the main test workflow into its own GitHub Actions workflow, so routine test runs aren’t marked as failed due to advisory DB updates or unrelated audit findings.

Changes:

• Removed the bundle-audit step from .github/workflows/test.yml.
• Added a new .github/workflows/audit.yml workflow that runs gem auditing only when Gemfile/Gemfile.lock changes, plus a weekly scheduled run.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
.github/workflows/test.yml	Removes gem auditing from the test workflow so tests aren’t blocked by audit results.
.github/workflows/audit.yml	Introduces a dedicated gem audit workflow triggered by Gemfile changes and a weekly schedule.

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