fix: retry transient streaming transport failures

[mikehostetler]

Summary

• add a dedicated streaming retry wrapper around Finch.stream/5
• retry transient transport failures only before any response body data has been emitted
• cover retryable, non-retryable, and partial-stream failure cases

Closes #314.

Verification

• mix test test/req_llm/streaming/retry_test.exs test/req_llm/streaming/finch_client_test.exs test/req_llm/integration/streaming_orchestration_test.exs
• mix test
• mix quality