deriveModelIdentifier

Produce a non-blank modelIdentifier for an ksl.app.config.optimization.OptimizationProblemSpec.

Order of preference:

1. descriptor.modelIdentifier (when available — that's the identifier of the actual Model the runtime will build).

2. A natural identifier derived from the model reference ("bundleId:modelId" for ByBundleAndModelId, the providerId for ByProviderId, etc.).

Returns null only when modelReference is null. The OptimizationProblemSpec init accepts a null field but rejects a blank string.