fix: support raw JSON response tool arguments

[lyz52789]

What changed

• preserve Responses API tool-call arguments as raw JSON instead of requiring a JSON string
• normalize both object-form and string-form arguments when converting Responses output to Chat Completions
• add regression coverage for response.output_item.done.item.arguments and response.completed.response.output[].arguments

Why

Some OpenAI-compatible upstreams return function-call arguments as a JSON object. The existing string DTO rejects those events during stream decoding with errors such as:

json: cannot unmarshal object into Go struct field ResponsesOutput.item.arguments of type string

This ports the focused upstream fix from QuantumNous/new-api commit db89b57e1c8ce27dcd38273a4791f806da789a57 and adds an end-to-end DTO regression test for both observed stream locations.

Independence

This commit is based directly on the current origin/main commit 738bbf25fdb42a46f891e3a80d4b18b6f9b275f3. It was tested from a clean detached worktree with no other local or upstream patches applied.

Validation

go test ./common ./dto ./service/openaicompat ./relay/channel/openai -count=1

All targeted packages pass from the clean worktree.

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