fix: ensure ResponseUsage token fields are int, not None (fixes #1179) (#1181)

### Problem

When using streaming responses, some models or API endpoints may return
`usage` fields (`prompt_tokens`, `completion_tokens`, `total_tokens`) as
`None` or omit them entirely. The current implementation passes these
values directly to the `ResponseUsage` Pydantic model, which expects
integers. This causes a validation error:

3 validation errors for ResponseUsage
input_tokens
Input should be a valid integer [type=int_type, input_value=None,
input_type=NoneType]
output_tokens
Input should be a valid integer [type=int_type, input_value=None,
input_type=NoneType]
total_tokens
Input should be a valid integer [type=int_type, input_value=None,
input_type=NoneType]

### Solution

This PR ensures that all token fields passed to `ResponseUsage` are
always integers. If any of the fields are `None` or missing, they
default to `0`. This is achieved by using `or 0` and explicit `is not
None` checks for nested fields.

**Key changes:**
- All `input_tokens`, `output_tokens`, `total_tokens` fields use `or 0`
fallback.

### Impact

- Fixes Pydantic validation errors for streaming responses with
missing/None usage fields.
- Improves compatibility with OpenAI and third-party models.
- No breaking changes; only adds robustness.


fixes #1179

Co-authored-by: thomas <[email protected]>

Author: kobol

src/agents/models/chatcmpl_stream_handler.py

@@ -493,9 +493,9 @@ async def handle_stream(
         final_response.output = outputs
         final_response.usage = (
             ResponseUsage(
-                input_tokens=usage.prompt_tokens,
-                output_tokens=usage.completion_tokens,
-                total_tokens=usage.total_tokens,
+                input_tokens=usage.prompt_tokens or 0,
+                output_tokens=usage.completion_tokens or 0,
+                total_tokens=usage.total_tokens or 0,
                 output_tokens_details=OutputTokensDetails(
                     reasoning_tokens=usage.completion_tokens_details.reasoning_tokens
                     if usage.completion_tokens_details