fix(cli): count critical Markdoc errors in validate

[tojacob03]

Summary

evidence validate returned valid: true and exited 0 when a page had a critical Markdoc error, such as an undefined tag.

Markdoc gives structural errors (tag-undefined, missing-opening, tag-placement-invalid, …) the level critical. The validate route only counted error and warning, so these errors were counted as neither. cli/cli/validate.ts already prints critical entries as errors, and serve shows them as a page-level error box. This change makes the JSON counts and the exit code agree with both.

Fixes #3340

Changes

• cli/src/routes/api/validate/+server.ts: count critical as an error.
• New validate-route.test.ts next to the route. It runs GET against a temporary project in syntax-only mode, the same way home-page-load.test.ts does, and checks:
  • a clean page is valid: true
  • {% unknown_tag /%} gives errorCount: 1 and valid: false

Testing

• The new test fails on main (errorCount is 0) and passes with the fix.
• cli Vitest suite: 389/389 passed.
• svelte-check on cli: the only error is the missing docs-bundle.generated module, which is created by the bun build step. It is not related to this change.

[vercel]

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[hughess]

/upstream

[github-actions]

Imported for internal review, this PR will be updated when it merges.

[hughess]

Thanks for the contribution, @tojacob03! Merged upstream, and synced back in 80a719c

[tojacob03]

Thanks for the quick review and merge, @hughess !