test: expand streaming core coverage

[mikehostetler]

Summary

• add streaming orchestration, fixture, SSE, websocket, and stream response coverage tests
• expand Finch streaming transport tests with live local streaming and error-path coverage
• raise the batch-02 target files above 75% coverage with tests only

Coverage

• total coverage: 63.3% -> 64.3%
• lib/req_llm/streaming.ex: 74.6% -> 78.8%
• lib/req_llm/streaming/finch_client.ex: 51.0% -> 82.6%
• lib/req_llm/streaming/fixtures.ex: 56.7% -> 97.2%
• lib/req_llm/streaming/sse.ex: 35.1% -> 86.4%
• lib/req_llm/streaming/web_socket_client.ex: 49.3% -> 77.3%
• lib/req_llm/stream_response.ex: 74.4% -> 94.1%

Verification

• mix test test/req_llm/streaming_test.exs test/req_llm/streaming/finch_client_test.exs test/req_llm/streaming/fixtures_test.exs test/req_llm/streaming/web_socket_client_test.exs test/req_llm/streaming/sse_test.exs test/req_llm/stream_response_test.exs
• MIX_ENV=test mix coveralls.json