refactor: switch test fixtures to use external runtime package

Configure all test fixtures (except parameters/) to import the
generated runtime package instead of inlining runtime code. This
reduces bloat in generated test files and exercises the external
runtime code path.

The parameters/ tests remain in inline mode to maintain coverage
of the extract+DCE pipeline.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mromaszewicz

experimental/codegen/internal/test/README.md

@@ -0,0 +1,10 @@
+# oapi-codegen internal tests
+
+Please note, we have a number of directories here, testing various topics.
+Try to find an existing topic to extend when making new tests, or if the
+topic is unrelated to what we have, make a new one.
+
+Wherever tests depend on the runtime package, we import our pre-generated
+runtime package from the repo root, except parameter tests, where
+we generate it inline, since we want to test that code path and dead code elimination,
+both of which are most heavily exercised in parameters code.

experimental/codegen/internal/test/callbacks/config.yaml

@@ -4,3 +4,5 @@ generation:
   callback-initiator: true
   callback-receiver: true
   server: std-http
+  runtime-package:
+    path: github.com/oapi-codegen/oapi-codegen-exp/experimental/runtime

experimental/codegen/internal/test/callbacks/output/callbacks.gen.go

@@ -0,0 +1,5 @@
+package: output
+output: output/types.gen.go
+generation:
+  runtime-package:
+    path: github.com/oapi-codegen/oapi-codegen-exp/experimental/runtime

experimental/codegen/internal/test/components/composition/config.yaml

@@ -2,4 +2,4 @@
 // oneOf/anyOf patterns, enums, readOnly/writeOnly, and x-go-name.
 package composition
 
-//go:generate go run ../../../../../cmd/oapi-codegen -package output -output output/types.gen.go spec.yaml
+//go:generate go run ../../../../../cmd/oapi-codegen -config config.yaml spec.yaml