Add fallback_on_response parameter to FallbackModel

[sarth6]

Summary

• Add fallback_on_response parameter to FallbackModel for response-based fallback
• Support streaming with response-based fallback via BufferedStreamedResponse
• Add 15 tests covering both non-streaming and streaming scenarios

Motivation

Closes #3640

When using built-in tools like web_fetch, a model may return a successful HTTP response (no exception) but the response content indicates a semantic failure. For example, Google's WebFetchTool returns URL_RETRIEVAL_STATUS_FAILED in the response rather than throwing an exception.

This PR adds a fallback_on_response callback that inspects the response content and triggers fallback when appropriate.

Changes

FallbackModel:

• New fallback_on_response: Callable[[ModelResponse, list[ModelMessage]], bool] | None parameter
• When set, the callback is invoked after each successful model response
• Returns True to reject the response and try the next model

Streaming support:

• When fallback_on_response is set with streaming, the entire response is buffered before evaluation
• BufferedStreamedResponse replays the buffered events to the caller
• Trade-off documented: caller won't receive partial results until full response is ready

Example

from pydantic_ai.models.fallback import FallbackModel

def web_fetch_failed(response: ModelResponse, messages: list[ModelMessage]) -> bool:
    for call, result in response.builtin_tool_calls:
        if call.tool_name == 'web_fetch':
            content = result.content
            if isinstance(content, dict):
                status = content.get('url_retrieval_status', '')
                if status and status != 'URL_RETRIEVAL_STATUS_SUCCESS':
                    return True
    return False

fallback_model = FallbackModel(
    google_model,
    anthropic_model,
    fallback_on_response=web_fetch_failed,
)

Test plan

• 9 non-streaming tests for fallback_on_response
• 6 streaming tests for fallback_on_response
• 100% test coverage maintained
• All 32 fallback tests pass

🤖 Generated with Claude Code