fix: accept object tool arguments in openai compat

[Alix-007]

Summary

• accept tool_calls.function.arguments as either a JSON string or a raw object
• preserve the raw serialized arguments on the parsed function call
• add regression coverage for both string and object argument payloads

Fixes #1287

Testing

• go test ./pkg/providers/openai_compat

[CLAassistant]

All committers have signed the CLA.

[yinwm]

Closing as duplicate of #1292, which has a more complete implementation:

• fix(openai_compat): accept object tool call arguments #1292 preserves the original arguments string via ToolCall.Function.Arguments
• fix(openai_compat): accept object tool call arguments #1292 adds Type field for API completeness
• fix(openai_compat): accept object tool call arguments #1292 has better test coverage

Both fix #1287, but #1292 is the better solution. Please refer to #1292 instead.