refactor(cms): convert page lifecycle and validators to errors-as-values [INTORG-716]

[Infi-Knight]

Summary

Sub #4 of INTORG-614. Converts the remaining CMS surfaces outside the sync pipeline (page lifecycle hooks and the two validators) to errors-as-values.

What's in the diff:

• pageLifecycle.ts:
  • fetchPublished returns PageData | null | Error. Distinguishes "no published version for this locale" (normal absence) from "Strapi document service threw" (real error). Callers in beforeUpdate and exportAllLocales narrow the three states explicitly.
  • writeMDXFile returns string | Error instead of throwing. exportAllLocales narrows on the result and logs continue-on-error so a single locale failure doesn't abort the rest.
  • shouldSkipMdxExport, getAdminAuthor, deleteMdxIfExists keep their try/catch fallback semantics with explanatory Why: comments. Their "errors" are legitimate "no request context" or "stale file" states, not failures the lifecycle hook should react to.
• validateFrontmatter.ts: ValidationError is now a real class extending Error instead of an interface. validateFrontmatter returns MDXFile | ValidationError (was ValidationError | null). validateMdxFiles narrows with instanceof.
• contentValidation.ts: validateNoNestedJsx returns errors.ValidationError | undefined instead of throwing. The Strapi middleware in cms/src/index.ts narrows on the return and translates to a 400 response. The try/catch around it is gone.
• Tests for contentValidation and validateFrontmatter updated to assert returned values.

Related Issue

INTORG-716

Manual Test

Run the CMS sync test suite:

```
pnpm --dir cms test:sync-mdx
```

Checks

• `pnpm run format`
• `pnpm run lint`

PR Checklist

• PR title follows Conventional Commits (e.g. `feat: ...`, `fix: ...`)
• Linked issue included
• Scope is focused (target ~10-20 files when possible)
• Screenshots for UI changes (if applicable)

[JonathanMatthey]

nicely split - lgtm