Fix Pydantic annotation metadata handling across 2.10-2.13

[Travior]

Fixes #3862 #3861

Summary

• Fix discriminated RootModel handling across Pydantic 2.10 → 2.13
• Preserve root-level Annotated[...] metadata when dlt rebuilds Pydantic models for schema contracts
• Add regressions for discriminated-union schema inference and root-metadata semantics

Problem 1 — Pydantic 2.13 moved where the discriminator lives

For tagged unions wrapped in a RootModel, Pydantic 2.13 moved the discriminator off the Annotated metadata of the root field and onto root_field.discriminator directly:

	root_field.annotation	root_field.discriminator
≤ 2.12	Annotated[Union[...], FieldInfo(discriminator="kind")]	None
≥ 2.13	Union[...] (bare)	"kind"

dlt assumed the discriminator was always recoverable from root_field.annotation, so on 2.13 discriminated root models were treated like a plain RootModel with a single root column, producing e.g. NOT NULL constraint failed: click_events.root.

This is partially fixed already, but only for the case where 2.13 strips the annotation down to a bare Union. It misses the case where root_field.annotation is still an Annotated[...] on 2.13 because additional metadata (e.g. an AfterValidator) is present — the discriminator lives on root_field.discriminator but the annotation is not bare, so the rescue path was skipped.

Problem 2 — dlt was corrupting Annotated[...] metadata

While rebuilding models for schema contracts, dlt did:

return Annotated[_process_annotation(a_t), tuple(a_m)]

This collapses N metadata entries into one tuple entry, changing Pydantic's semantics. For example:

Annotated[Union[ClickEvent, PurchaseEvent],
          Field(discriminator="kind"),
          AfterValidator(reject_forbidden_clicks)]

was effectively rebuilt as:

Annotated[Union[...], (FieldInfo(...), AfterValidator(...))]

For regular BaseModel this went unnoticed because create_model(__base__=model, ...) re-inherits the original, correctly-spread metadata. For RootModel there is no __base__ rescue (it's built with type(name, (PydanticRootModel[ann],), ...)), so validators and other root-level metadata were silently dropped.

Fix

• Support both discriminator representations:
  • older Pydantic: inside Annotated[..., FieldInfo(...)]
  • newer Pydantic: on root_field.discriminator, including when root_field.annotation is still Annotated due to other metadata
• Preserve root-model discriminator semantics when rebuilding contract-adjusted RootModel[...]
• Rebuild Annotated[...] metadata by preserving entries individually instead of packing them into a tuple

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[Travior]

To me it's unclear why we previously wrapped other annotation metadata into tuples, losing their semantics:
Annotated[str, FieldInfo(...), AfterValidator(...)]
would be turned into:
Annotated[str, (FieldInfo(...), AfterValidator(...)].

Pydantic then ignores this tuple and we effectively lose the validator (or other annotation metadata)

[rudolfix]

this is certainly a bug. I probably overlooked that in claude-generated code.

[rudolfix]

LGTM! it is very hard to verify Pydantic quirks - I added two more tests to have (hopefully) complete coverage - test_apply_contract_preserves_nested_annotated_metadata_entries checks tuple bug directly

[rudolfix]

this is certainly a bug. I probably overlooked that in claude-generated code.