[BUG](codegen) Union- or model-rooted RootModel entry points miscompile as scalar aliases

Summary

The #593 fix routes every RootModel entry point through the alias path: extract_model_spec returns None for any RootModel, and extract_alias_spec documents it as a NewTypeSpec over its bare root shape. That is correct only when the root is a scalar (or plain container). A RootModel whose root is a discriminated union or a model -- a valid Pydantic shape, RootModel[Annotated[Union[...], Field(discriminator=...)]] -- is silently miscompiled: it documents as a scalar type alias and is skipped from expression generation, when it should generate as a union (per-arm markdown, per-arm PySpark checks) exactly as the bare Annotated[Union] alias does today. There is no error; the output is just wrong.

Why it is latent

The Overture schema declares no union- or model-rooted RootModel entry points today, so nothing currently triggers this. It becomes a live defect the moment such a shape appears -- either an external user running the codegen on their own models, or the in-flight migration of Segment from a bare Annotated[Union] alias to RootModel (the more idiomatic Pydantic spelling for a validated root entity).

Root cause

Discovery classifies entry points by is it a RootModel? rather than by what is the RootModel's root?:

• extract_model_spec (spec_discovery.py:42): if is_rootmodel(entry): return None
• extract_alias_spec (spec_discovery.py:64): if is_rootmodel(entry): return extract_rootmodel_alias(entry)

Both branches fire for any RootModel regardless of root shape.

Related type-signature lie

discover_models is annotated -> ModelDict where ModelDict = dict[ModelKey, type[BaseModel]] (discovery/types.py:17), but a discriminated-union entry point stores an Annotated[Union[...], Field(discriminator=...)] alias, which is not a type[BaseModel]. The annotation is already inaccurate for union entry points. Migrating unions to RootModel makes it honest (a RootModel is a BaseModel subclass); short of that, the type should be widened to reflect what discovery actually yields.

Expectation

Condition the entry-point classification on the RootModel's root shape rather than on RootModel-ness:

• scalar/container root -> alias page, as today
• discriminated-union or model root -> generate as a union/model (peel the RootModel, extract the inner shape), not an alias

At minimum, a union- or model-rooted RootModel entry point must not be silently documented as a scalar alias. If full support is not built in the same change, detect and raise, matching the self-referential-RootModel precedent from #593.

Related

Follow-up gap in the #593 fix.