Fix incorrect OpenAPI definition when using FromForm with IEnumerable

[sarbas33]

Fix incorrect OpenAPI schema for single [FromForm] collections

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Wrap single [FromForm] IEnumerable parameters in an object schema.

Description
This PR fixes a bug in the OpenAPI document generation where a single IEnumerable parameter decorated with [FromForm] was incorrectly serialized as a raw JSON array at the root of the request body.

Because application/x-www-form-urlencoded and multipart/form-data are key-value based formats, a collection must be associated with a field name (key). Previously, the logic identified IEnumerable types as "Complex Types" but failed to recognize that they cannot function as the root schema of a form, unlike POCOs.

Changes:

Added logic to GetFormRequestBody to detect IEnumerable types (excluding string).

Ensured that these collection types are always treated as properties of the form object rather than being assigned to the root complexTypeSchema.

Fixes #62328

[dotnet-policy-service]

Thanks for your PR, @@sarbas33. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[sarbas33]

@dotnet-policy-service agree