test: add parameter roundtrip test infrastructure

Add a full client/server roundtrip test for all parameter styles,
locations, and explode combinations. The test generates a client and
std-http server from a shared spec, sends requests through the
generated client, and verifies the server echoes back the correct
values.

NOTE: Tests do not compile in this commit. Known blocking issues:
- BindFormParam template passes (bool, url.Values) instead of (ParamLocation, string) for query params
- BindFormExplodeParam template passes (ParamLocation, string) instead of (bool, url.Values) for cookie params
- BindDeepObjectExplodeParam and StyleDeepObjectExplodeParam missing from V3 runtime
- Unused err variable in passthrough path handler template

Author: mromaszewicz-nv

codegen/internal/test/parameters/roundtrip/client/client.cfg.yaml

@@ -0,0 +1,4 @@
+package: client
+output: client.gen.go
+generation:
+  client: true