Apply BackendAIAPIMeta to REST API DTOs

## Motivation

Apply `BackendAIAPIMeta` annotations to REST API request/response DTOs for consistent documentation and version tracking.

## Scope

Update files in:

- `src/ai/backend/manager/api/*/types.py`
- `src/ai/backend/manager/dto/*.py`

Apply metadata pattern:

```python
class CreateSessionRequest(BaseModel):
name: Annotated[
str,
Field(min_length=1, max_length=64),
BackendAIAPIMeta(
description="Session name.",
added_version="25.1.0",
example="my-session",
),
]

config: Annotated[
SessionConfig,
Field(),
BackendAIAPIMeta(
description="Session configuration.",
added_version="25.1.0",
composite=True,
),
]
```

## Acceptance Criteria

- All REST API DTOs annotated with BackendAIAPIMeta
- Primitive fields have explicit example values
- Composite fields use `composite=True` for auto-generation
- API documentation reflects metadata

## Dependencies

- BA-3784 (metadata classes implementation)

## References

- [BEP-1022: Pydantic Field Metadata Annotation](https://github.com/lablup/backend.ai/blob/main/proposals/BEP-1022-pydantic-field-annotations.md) Phase 3

JIRA Issue: BA-3787