feat(responses): add usage types to inference and responses APIs (#3764)

## Summary
Adds OpenAI-compatible usage tracking types to enable reporting token
consumption for both streaming and non-streaming responses.

## Type Definitions
**Chat Completion Usage** (inference API):
```python
class OpenAIChatCompletionUsage(BaseModel):
    prompt_tokens: int
    completion_tokens: int
    total_tokens: int
    prompt_tokens_details: OpenAIChatCompletionUsagePromptTokensDetails | None
    completion_tokens_details: OpenAIChatCompletionUsageCompletionTokensDetails | None
```

**Response Usage** (responses API):
```python
class OpenAIResponseUsage(BaseModel):
    input_tokens: int
    output_tokens: int
    total_tokens: int
    input_tokens_details: OpenAIResponseUsageInputTokensDetails | None
    output_tokens_details: OpenAIResponseUsageOutputTokensDetails | None
```

This matches OpenAI's usage reporting format and enables PR #3766 to
implement usage tracking in streaming responses.

Co-authored-by: Claude <noreply@anthropic.com>

llama_stack/apis/agents/openai_responses.py

@@ -346,6 +346,42 @@ class OpenAIResponseText(BaseModel):
     format: OpenAIResponseTextFormat | None = None
 
 
+class OpenAIResponseUsageOutputTokensDetails(BaseModel):
+    """Token details for output tokens in OpenAI response usage.
+
+    :param reasoning_tokens: Number of tokens used for reasoning (o1/o3 models)
+    """
+
+    reasoning_tokens: int | None = None
+
+
+class OpenAIResponseUsageInputTokensDetails(BaseModel):
+    """Token details for input tokens in OpenAI response usage.
+
+    :param cached_tokens: Number of tokens retrieved from cache
+    """
+
+    cached_tokens: int | None = None
+
+
+@json_schema_type
+class OpenAIResponseUsage(BaseModel):
+    """Usage information for OpenAI response.
+
+    :param input_tokens: Number of tokens in the input
+    :param output_tokens: Number of tokens in the output
+    :param total_tokens: Total tokens used (input + output)
+    :param input_tokens_details: Detailed breakdown of input token usage
+    :param output_tokens_details: Detailed breakdown of output token usage
+    """
+
+    input_tokens: int
+    output_tokens: int
+    total_tokens: int
+    input_tokens_details: OpenAIResponseUsageInputTokensDetails | None = None
+    output_tokens_details: OpenAIResponseUsageOutputTokensDetails | None = None
+
+
 @json_schema_type
 class OpenAIResponseObject(BaseModel):
     """Complete OpenAI response object containing generation results and metadata.
@@ -363,6 +399,7 @@ class OpenAIResponseObject(BaseModel):
     :param text: Text formatting configuration for the response
     :param top_p: (Optional) Nucleus sampling parameter used for generation
     :param truncation: (Optional) Truncation strategy applied to the response
+    :param usage: (Optional) Token usage information for the response
     """
 
     created_at: int
@@ -380,6 +417,7 @@ class OpenAIResponseObject(BaseModel):
     text: OpenAIResponseText = OpenAIResponseText(format=OpenAIResponseTextFormat(type="text"))
     top_p: float | None = None
     truncation: str | None = None
+    usage: OpenAIResponseUsage | None = None
 
 
 @json_schema_type