gha cleanups (#6275)

# Description of Changes

Adds all pre commit PR checks under single file with job validator to
have a true "required" run step "all-checks-passed" check
Moves all GHAs into workflow helper function calls 

Note if: always() overrides the default skip-on-needs-failure, "Needs"
is added to ensure the validation runs at end of all other tasks

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

.github/workflows/ai-engine.yml

@@ -1,11 +1,12 @@
 name: AI Engine CI
 
+# Validates the Python AI engine: regenerates tool models, runs fixers,
+# lint, type-check, and tests. Called from build.yml on PRs and merge_group;
+# also runs directly on push to main as a post-merge safety net.
 on:
+  workflow_call:
   push:
     branches: [main]
-  pull_request:
-  merge_group:
-    branches: [main]
 
 permissions:
   contents: read