### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When streaming via the Anthropic chat handler, ModelResponseIterator.chunk_parser() raises KeyError: 'text' at litellm/llms/anthropic/chat/handler.py:789 whenever an upstream provider emits a content_block_start event for a text block without the optional text field.

The Anthropic Messages streaming spec says the initial text on content_block_start is conventionally "" (https://docs.anthropic.com/en/api/messages-streaming#content-block-events), and some Anthropic-compatible reverse-proxy providers (observed against api-cc.freemodel.dev proxying claude-haiku-4-5-20251001) ship the field unset. The actual content arrives via subsequent content_block_delta chunks regardless, so the field on content_block_start is effectively informational.

Because LiteLLM aborts the stream at this point, callers see a MidStreamFallbackError(litellm.APIConnectionError: 'text') and the model group gets cooled down — even though the request is otherwise healthy.

### Steps to Reproduce

Minimal: feed a chunk that mimics the upstream wire format into the parser dire…