Add Promote/Rollback GQL mutations

## Overview

Create dedicated GraphQL mutations for revision promotion and rollback operations.

**Note:** Currently `updateModelDeployment` with `activeRevisionId` can change the active revision, but dedicated mutations provide better UX and can include validation logic.

## Tasks

### 1. Implement `promoteDeploymentRevision` mutation

Promote a specific revision to be the active revision.

```
input PromoteRevisionInput {
deploymentId: ID!
revisionId: ID!
}

type PromoteRevisionPayload {
deployment: ModelDeployment!
previousRevisionId: ID
promotedRevisionId: ID!
}

mutation {
promoteDeploymentRevision(input: PromoteRevisionInput!): PromoteRevisionPayload
}
```

**Logic:**

- Validate target revision exists and belongs to the deployment
- Validate revision is within `revision_history_limit`
- Set `endpoint.current_revision` to target revision
- Update `deployment_state` appropriately
- Handle traffic_status transitions for routes

### 2. Implement `rollbackDeployment` mutation

Rollback to a previous revision.

```
input RollbackDeploymentInput {
deploymentId: ID!
# One of the following must be provided:
stepsBack: Int # Number of revisions to go back (e.g., 1 = previous)
targetRevisionId: ID # Specific revision to rollback to
}

type RollbackDeploymentPayload {
deployment: ModelDeployment!
rolledBackFromRevisionId: ID
rolledBackToRevisionId: ID!
}

mutation {
rollbackDeployment(input: RollbackDeploymentInput!): RollbackDeploymentPayload
}
```

**Logic:**

- Validate target revision (by steps_back calculation or direct ID)
- Validate revision exists in history
- Set `current_revision` to target
- Update deployment_state to indicate rollback in progress
- Respect deployment policy (rolling_update vs blue_green)

## File Location

Add to `src/ai/backend/manager/api/gql/model_deployment/model_deployment.py` or create new `deployment_operations.py`

## Acceptance Criteria

- [ ] `promoteDeploymentRevision` mutation with validation
- [ ] `rollbackDeployment` mutation with steps_back and targetRevisionId support
- [ ] Proper error handling for invalid revisions
- [ ] Unit tests for both mutations

JIRA Issue: BA-3429