Protocol bridge helpers.
This crate owns gateway/protocol conversion code that can be consumed by
both the siumai facade and siumai-extras without moving provider-specific
bridge logic into siumai-core or creating a facade/extras dependency cycle.
The current focus is explicit request bridging with reusable internal structure:
- planner
- request primitives
- request pair modules
- request inspection
- request serialization
- response inspection / serialization
- stream inspection / serialization