fix(openai_compat): accept object tool call arguments

[qs3c]

Summary

• accept OpenAI-compatible tool call arguments returned as either JSON strings or JSON objects
• preserve the existing raw fallback/logging path for malformed or unsupported payloads
• add a regression test for object-form tool call arguments

Fixes #1287.

Testing

• go test ./pkg/providers/openai_compat
• go test ./pkg/providers/... (fails on existing CodexCliProvider PATH-based tests on Windows; unrelated to this change)

[nikolasdehor]

Solid fix for handling providers that return tool call arguments as JSON objects instead of JSON strings.

Review notes:

1. decodeToolCallArguments -- the function handles all three cases correctly:

   • null/empty: returns empty map
   • string (standard OpenAI format): unmarshals the string as JSON
   • object (non-standard but valid): uses the map directly
   • other types: logs and falls back to raw string

2. json.RawMessage for Arguments field -- this is the correct type to use when the wire format can be either a string or an object. The previous string type would fail to unmarshal object-form arguments.

3. Graceful fallback -- malformed or unsupported argument types still produce a result with a "raw" key, preserving debuggability. This matches the previous behavior.

4. Test coverage -- the new test TestProviderChat_ParsesToolCallsWithObjectArguments creates a realistic server response with object-form arguments and verifies both string and boolean argument values are preserved.

5. Minor: the bytes.TrimSpace call on raw before checking for "null" is good defensive coding -- some providers may return " null " with whitespace.

LGTM.

[tsterbak]

What is missing for this PR to get merged?

Would be great to get it working again :)

[yinwm]

Reviewed and compared with #1379. This PR is more complete:

• Preserves original arguments string via ToolCall.Function.Arguments
• Adds Type field for completeness
• Better test coverage including Function field validation

The other PR (#1379) loses the raw arguments string which may be needed downstream.

LGTM 👍