fix(adk): serialize nil FunctionCall.Args as {} not null for OpenAI adapters (#2851)

[boxcee-interview]

Closes #2851

What was wrong

A zero-argument tool call succeeds in one task, then fails on the follow-up task after history is reloaded. Persistence drops empty function-call args (FunctionCall.Args is json:"args,omitempty"), so a reloaded call carries a nil Args map. The OpenAI chat-completions and Responses adapters then serialized that nil map as the string "null" instead of "{}". Object-validating backends (oMLX/ollama) reject it with HTTP 422.

What the fix does

Coerce a nil Args map to an empty map before marshaling, in the two OpenAI adapters that serialize function-call arguments:

• go/adk/pkg/models/openai_adk.go (chat completions)
• go/adk/pkg/models/openai_responses.go (Responses API)

This mirrors the identical nil-to-empty coercion already present in the Bedrock adapter (bedrock.go:686). Added two regression tests (one per adapter) asserting a nil-args call serializes to a JSON object, not null.

How to Test

Docker, golang:1.27, module root go/:

go test ./adk/pkg/models/ -run 'NilFunctionCallArgs' -v

Both tests PASS. Full package: go test ./adk/pkg/models/ → ok.

=== RUN   TestGenaiContentsToOpenAIMessages_NilFunctionCallArgs
--- PASS: TestGenaiContentsToOpenAIMessages_NilFunctionCallArgs (0.00s)
=== RUN   TestGenaiContentsToResponsesInput_NilFunctionCallArgs
--- PASS: TestGenaiContentsToResponsesInput_NilFunctionCallArgs (0.00s)

[supreme-gg-gg]

This is a duplicate of #2852, pls check for existing PRs before opening a new one next time