fix(builder): Defer graph validation to backend (#10556)

- Resolves #10553

### Changes 🏗️

- Remove frontend graph validation in `useAgentGraph:saveAndRun(..)`
  - Remove now unused `ajv` dependency
- Implement graph validation error propagation (backend->frontend)
  - Add `GraphValidationError` type in frontend and backend
  - Add `GraphModel.validate_graph_get_errors(..)` method
  - Fix error handling & propagation in frontend API request logic

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] Saving & running a graph with missing required inputs gives a
node-specific error
- [x] Saving & running a graph with missing node credential inputs
succeeds with passed-in credentials

autogpt_platform/frontend/package.json

@@ -56,7 +56,6 @@
     "@tanstack/react-table": "8.21.3",
     "@types/jaro-winkler": "0.2.4",
     "@xyflow/react": "12.8.2",
-    "ajv": "8.17.1",
     "boring-avatars": "1.11.2",
     "class-variance-authority": "0.7.1",
     "clsx": "2.1.1",