Stream SSE responses and improve response body errors

[ryanfowler]

Summary

• Stream text/event-stream responses through the formatted output path so SSE events print as they arrive instead of waiting for EOF.
• Preserve SSE parsing across chunk boundaries and update the SSE example to match the formatter’s actual output.
• Replace the vague request or response body error surface with a clearer response body error: ... message that keeps useful transport detail.
• Add integration coverage for live SSE streaming and partial-body failures.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test integration -- --test-threads=1