[ModelDeploy] Guard the preset-derived replica count and strategy fallbacks

## Goal

Close two unguarded fallbacks in `DeploymentController._apply_deployment_level_preset` that are safe only by accident.

## Background

Both are in `sokovan/deployment/deployment_controller.py`, on the path every preset-driven deploy takes.

**Null replica count.** `:773-780` reads the preset's replica count with no null check:

```python
preset_replica = preset_data.replica_count if preset_data is not None else self._DEFAULT_REPLICA_COUNT
resolved_replica_spec = ReplicaSpec(replica_count=preset_replica)
```

The `open_to_public` fallback immediately below (`:782-789`) does guard with `is not None` and falls through to the system default. The replica branch only survives because `DeploymentRevisionPresetData.replica_count` is a non-optional `int` today; the asymmetry means the intent is not enforced, and a null would reach `endpoints.replicas` as an integrity error at insert time.

**Non-exhaustive strategy match.** `_build_policy_from_preset` (`:803-824`) matches `DeploymentStrategy.ROLLING` and `BLUE_GREEN` with no fallback arm, so `strategy_spec` would be unbound if `deployment_strategy` ever became nullable or gained a member. The failure mode is an `UnboundLocalError`, not a domain error.

Neither is reachable today. Both are one-line fixes and both sit on the code path this epic is already touching.

## Acceptance Criteria

- The replica fallback chain is explicit: caller input, then preset value when present, then `_DEFAULT_REPLICA_COUNT` — matching the shape of the `open_to_public` branch beside it.
- `_build_policy_from_preset` handles every `DeploymentStrategy` member, with any unhandled case surfacing as a domain error rather than an `UnboundLocalError`.

JIRA Issue: BA-XXXX

Start in sokovan/deployment/deployment_controller.py and read _apply_deployment_level_preset alongside the guarded open_to_public fallback. Then inspect _build_policy_from_preset and the DeploymentStrategy members; done means the replica fallback reaches _DEFAULT_REPLICA_COUNT when needed and every strategy path produces a domain error rather than an unbound variable.

Written by the indexing model from the issue text.