Implement backend_ai_field() utility for Strawberry GraphQL

## Objective

Implement the core utility functions defined in BEP-1031 for integrating `BackendAIAPIMeta` with Strawberry GraphQL.

## Deliverables

- [ ] Implement `backend_ai_field()` wrapper function
- [ ] Implement `backend_ai_type()` decorator
- [ ] Implement `backend_ai_input()` decorator
- [ ] Implement metadata extraction utilities (`get_gql_field_meta()`, `collect_all_field_versions()`)
- [ ] Add unit tests
- [ ] Update `src/ai/backend/manager/api/gql/README.md` with usage examples

## Implementation Location

`src/ai/backend/manager/api/gql/utils.py`

## Related

- Epic: BA-3928
- BEP: BEP-1031 (GraphQL API Field Metadata Extension)
- Related BEP: BEP-1022 (Pydantic Field Metadata Annotation)

JIRA Issue: BA-3930