Bound SSE formatted stream buffer growth

[ryanfowler]

Summary

• Add bounded pending-buffer handling to the SSE formatter so unterminated lines and events cannot grow without limit
• Thread the new fallible SSE formatter API through the HTTP streaming path and reuse the existing response buffer ceiling
• Add unit coverage for oversized pending input and for counter reset after event dispatch

Testing

• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1