Pluggable HTTP server abstractions improvements

[RomanEmreis]

Summary

• HttpEngine::adapt_request no longer forces engines to .unwrap()/.expect()
• HttpEngine::adapt_response drops the BytesMut round-trip for the default Streamable HTTP implementation.
• parse_message single-step decode + Error::classify(). Drops the serde::Value intermediate from the single-message hot path.
• Removed 'static constraint for HttpEngine::Request, HttpEngine::Response and HttpEngine::SseEvent

Type

• Bug fix
• Feature
• Enhancement
• Performance
• Documentation
• Refactor
• Security
• Breaking change

Checklist

• I added/updated tests where it makes sense
• I updated docs/examples if needed
• This change is backwards-compatible (or clearly marked as breaking)
• I ran formatting/lints locally (if applicable)