fix: enable failover during stream iteration

[kachelle]

Summary

Streaming failover doesn't work because withModelFailover() only catches FailoverableException during the synchronous StreamableAgentResponse creation, not during the lazy generator iteration where HTTP errors
actually occur.

Problem

Promptable::stream() calls withModelFailover(), which wraps a try-catch around:

return $callback($provider, $model); // returns StreamableAgentResponse immediately

This succeeds instantly because StreamableAgentResponse uses a lazy generator - no HTTP call is made until the stream is iterated. By the time a provider returns 429 or 529 (during foreach iteration), execution is
outside the failover try-catch.

Passing provider: [Lab::Mistral, Lab::Anthropic] to stream() never actually fails over.

Solution

When multiple providers are configured, stream() now builds a StreamableAgentResponse with a failover-aware generator. The generator uses yield from to iterate each provider's stream, catching FailoverableException
during iteration and trying the next provider.

Single-provider calls use a direct path with no wrapping overhead.

Tests

• test_stream_fails_over_to_next_provider_when_primary_is_rate_limited - verifies failover works and AgentFailedOver event is dispatched

• test_stream_throws_last_exception_when_all_providers_fail - verifies the last exception is thrown when no provider succeeds

  Note: This fix works in conjunction with providers correctly classifying HTTP errors as FailoverableException. See feat: map Mistral HTTP 503 to PrismProviderOverloadedException prism-php/prism#959 for related work on mapping Mistral 503 responses.

This is only my 2nd PR to this project, happy to adjust the approach if needed.