bug: nil pointer dereference in Flux plugin when NumProcPerNode is unset

## What happened?

`generateFluxEntrypoint` in `pkg/runtime/framework/plugins/flux/flux.go` unconditionally dereferences `info.RuntimePolicy.MLPolicySource.Flux.NumProcPerNode` without a nil check.

The field has a `+kubebuilder:default=1` CRD schema default, but that default is only applied by the API server when the object is admitted. Objects constructed directly in code (unit tests, or a runtime snapshot loaded before the default existed) may have this field unset, causing a nil pointer dereference panic in `generateFluxEntrypoint`.

## Affected code

`pkg/runtime/framework/plugins/flux/flux.go`, inside `generateFluxEntrypoint`:

```go
// before fix — panics if NumProcPerNode is nil
tasks = *info.RuntimePolicy.MLPolicySource.Flux.NumProcPerNode
```

## What should happen?

The code should fall back to the documented default of 1 when the field is nil, matching the field's own CRD schema intent:

```go
tasks = ptr.Deref(info.RuntimePolicy.MLPolicySource.Flux.NumProcPerNode, 1)
```

## How to reproduce

Run the new regression test added in the accompanying PR with the fix temporarily reverted — it panics with a nil dereference inside `buildInitScriptConfigMap`.

## Why existing tests did not catch it

All existing test cases in `TestOptionalTrainerFields` hardcoded `NumProcPerNode: ptr.To[int32](1)` in the runtime fixture, so the nil path was never exercised.

Note: This contribution was developed with AI assistance (Claude), in accordance with the Kubeflow AI Policy.

Start in pkg/runtime/framework/plugins/flux/flux.go at generateFluxEntrypoint, then inspect TestOptionalTrainerFields and run the Flux plugin package tests. Done means a runtime policy with NumProcPerNode unset no longer panics and uses the documented default of 1.

Written by the indexing model from the issue text.