Summary

Fix Pydantic query/header/cookie models so they still flatten when there are sibling params in the same location.

Problem

FastAPI only flattened these models when they were the only param there:

• Annotated[MyModel, Query()]
• Annotated[MyModel, Header()]
• Annotated[MyModel, Cookie()]

If you added a sibling param like extra: Query() or extra: Header(), the model stopped flattening. That broke request parsing, validation errors, and OpenAPI docs.

Fix

Model-backed query/header/cookie params now flatten consistently, even with sibling params.

This keeps existing behavior for the single-model case and preserves:

• aliases / validation aliases
• list handling
• header underscore conversion
• body embedding behavior

Old vs New

Before:

def read_items(
    m: Annotated[Filters, Query()],
    extra: Annotated[int, Query()],
)

FastAPI could treat m like one query param instead of flattening it into limit and q.

After:

• limit, q, and extra are all handled as separate query params
• validation errors point to the individual fields
• OpenAPI shows flattened params, not m

Same fix applies to Header() and Cookie().