Standardize reasoning field to OpenAI format while maintaining input compatibility (#3136)

* Initial plan

* Add comprehensive reasoning field standardization tests

Co-authored-by: hlohaus <983577+hlohaus@users.noreply.github.com>

* Standardize reasoning field to OpenAI format while maintaining input compatibility

Co-authored-by: hlohaus <983577+hlohaus@users.noreply.github.com>

* Rename reasoning_content parameter to reasoning for consistent naming

Co-authored-by: hlohaus <983577+hlohaus@users.noreply.github.com>

* Address review comments: remove hardcoded path and rename reasoning_content to reasoning

Co-authored-by: hlohaus <983577+hlohaus@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: hlohaus <983577+hlohaus@users.noreply.github.com>

docs/reasoning-standardization.md

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+# Reasoning Field Standardization
+
+## Issue
+DeepSeek uses `"reasoning_content"` field while OpenAI uses `"reasoning"` field in their chat completion streaming responses. This inconsistency caused confusion about what field name to use in the g4f Interference API.
+
+## Decision
+**Standardized on OpenAI's `"reasoning"` field format for API output while maintaining input compatibility.**
+
+## Rationale
+1. **OpenAI Compatibility**: OpenAI is the de facto standard for chat completion APIs
+2. **Ecosystem Compatibility**: Most tools and libraries expect OpenAI format
+3. **Consistency**: Provides a unified output format regardless of the underlying provider
+4. **Backward Compatibility**: Input parsing continues to accept both formats
+
+## Implementation
+
+### Input Format Support (Unchanged)
+The system continues to accept both input formats in `OpenaiTemplate.py`:
+```python
+reasoning_content = choice.get("delta", {}).get("reasoning_content", choice.get("delta", {}).get("reasoning"))
+```
+
+### Output Format Standardization (Changed)
+- **Streaming Delta**: Uses `reasoning` field (OpenAI format)
+- **Non-streaming Message**: Uses `reasoning` field (OpenAI format)  
+- **API Responses**: Should use standard OpenAI streaming format
+
+### Example Output Formats
+
+#### Streaming Response (OpenAI Compatible)
+```json
+{
+  "id": "chatcmpl-example",
+  "object": "chat.completion.chunk",
+  "choices": [{
+    "index": 0,
+    "delta": {
+      "role": "assistant",
+      "reasoning": "I need to think about this step by step..."
+    },
+    "finish_reason": null
+  }]
+}
+```
+
+#### Non-streaming Response
+```json
+{
+  "choices": [{
+    "message": {
+      "role": "assistant",
+      "content": "Here's my answer",
+      "reasoning": "My reasoning process was..."
+    }
+  }]
+}
+```
+
+## Files Changed
+- `g4f/client/stubs.py`: Updated to use `reasoning` field instead of `reasoning_content`
+
+## Testing
+- Added comprehensive tests for format standardization
+- Verified input compatibility with both OpenAI and DeepSeek formats
+- Confirmed no regressions in existing functionality