OpenAPI 3.0's deepObject style has undefined behavior for arrays.
Two conventions exist in the wild:

- Indexed: p[vals][0]=a&p[vals][1]=b (oapi-codegen's current format)
- Non-indexed: p[vals]=a&p[vals]=b (Swagger UI / Rails convention)

Swagger UI generates the non-indexed format, which previously failed
with "[field] has multiple values". This change makes UnmarshalDeepObject
accept both formats by detecting repeated query parameter keys and
expanding them into synthetic indexed entries (e.g. [vals][0], [vals][1])
before feeding them into the existing tree-construction and assignment
logic.

Marshaling (MarshalDeepObject) intentionally remains unchanged and
continues to emit the indexed format. The indexed format is
unambiguous, already consumed correctly by all known implementations,
and is what oapi-codegen's own generated clients expect. Changing it
to the non-indexed format would be a breaking change for consumers
that rely on the current wire format, with no practical benefit since
both formats are now accepted on the unmarshaling side.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>