Revision add silently drops a partially specified resource config

## Problem

When adding a deployment revision, the adapter builds the ResourceSpec only if cluster_config and resource_config are both present. Supplying just one silently discards the other, and the request fails later with a message that points somewhere else.

```
# api/adapters/deployment/adapter.py, add_revision
resource_spec = None
if input.cluster_config is not None and input.resource_config is not None:
resource_spec = ResourceSpec(...)
```

## Reproduction

Add a revision with resource_config but no cluster_config:

```
./bai deployment revision add --config '{
"model_mount_config": {...}, "model_runtime_config": {...}, "image": {...},
"resource_config": {"resource_slots": {"entries": [
{"resource_type": "cpu", "quantity": "1"},
{"resource_type": "mem", "quantity": "2147483648"}]}}}'

InvalidAPIParameters: resource_slots is missing required resource slot(s): ['cpu', 'mem'].
```

The slots were supplied. They were dropped because cluster_config was absent, and the error names the field that was provided rather than the one that was missing. Verified against a live manager on 26.8.0rc1; it took six attempts to find the real cause.

## Expected

Both fields are optional on AddRevisionInput because a revision preset may supply them, unlike CreateRevisionInput where both are required. That makes silent discard the wrong behavior for a partially specified pair. The controller already rejects each missing merged piece explicitly - image_id, mounts, model_definition - so this path should say what is missing too.

## Acceptance Criteria

- Supplying only one of cluster_config / resource_config is rejected with a message naming the missing companion
- Supplying both keeps working unchanged
- Supplying neither keeps working, so a revision preset can still provide them
- CI quality gates (format, lint, type check, unit suite) are green

## References

- src/ai/backend/manager/api/adapters/deployment/adapter.py - add_revision
- src/ai/backend/manager/sokovan/deployment/deployment_controller.py - the explicit checks this path lacks
- CHECKLIST_26.8 item 12 - Model Serving / Deployment, found while running it

JIRA Issue: BA-7100

Start in src/ai/backend/manager/api/adapters/deployment/adapter.py at add_revision, then read the explicit merged-field checks in src/ai/backend/manager/sokovan/deployment/deployment_controller.py. Verify the three one-field, both-fields, and neither-fields cases, and run the format, lint, type-check, and unit-suite quality gates; done means the missing companion is named without changing valid cases.

Written by the indexing model from the issue text.