Schedule regular XML validation

[adunning]

Augment the existing XML validation by creating inline annotations on pull requests and validating the entire repository weekly to ensure it is up-to-date with any improvements to the schema.

[Copilot]

Pull request overview

Adds a GitHub Actions workflow to validate CSL (XML) files on PRs/pushes and to run a full-repository validation on a weekly schedule, helping keep the repository aligned with the upstream CSL schema.

Changes:

• Introduces a new Validate XML workflow triggered on PRs and pushes affecting **/*.csl.
• Adds a weekly scheduled run (and manual dispatch) that validates the full repository.

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[github-actions]

😟 There are some issues with your submission.

2 tests failed

dependent/adhd-attention-deficit-and-hyperactivity-disorders: must validate against the CSL 1.0.1 schema
Please check your style at http://validator.citationstyles.org/

expected `[[0, "ERROR: Element style failed to validate attributes"], [4, "4:0: ERROR: Expecting an element aut...ment info failed to validate content"], [2, "2:0: ERROR: Element style failed to validate content"]].empty?` to be truthy, got false

dependent/adhd-attention-deficit-and-hyperactivity-disorders: must be parsable as a CSL style

not allowed to add #<CSL::Info::Id "http://www.zotero.org/styles/adhd-attention-deficit-and-hyperactivity-disorders"> to #<struct CSL::Style::Children :"style-options"=nil, info=#<CSL::Info children=[2]>, locale=[], macro=[], citation=nil, bibliography=nil>

Please check the test report for details.