Centralize import/helper tracking via CodegenContext and add behavioral tests for helper templates

Replace the scattered ParamUsageTracker and per-phase import collection
with a single CodegenContext that all generators share. This eliminates
duplicate param tracker creation across client, server, webhook, and
callback code paths, and moves import registration, helper tracking,
param function tracking, and custom type tracking into one place.

Key changes to code generation pipeline:
- Add CodegenContext (codegen_context.go) as the centralized tracker for
  imports, helpers, param functions, and custom type templates
- Refactor Generate() in codegen.go into three phases: (1) generate all
  code sections, (2) render helpers/params/custom types, (3) assemble
  output — eliminating repeated param/form-helper generation per phase
- Remove ParamUsageTracker from paramgen.go; its logic is absorbed by
  CodegenContext.NeedParam(), GetRequiredParamTemplates(), etc.
- Update TypeGenerator to delegate imports and template tracking to ctx
- Update GatherOperations, GatherWebhookOperations, and
  GatherCallbackOperations to accept *CodegenContext instead of
  *ParamUsageTracker
- Move loadCustomType into CodegenContext.loadAndRegisterCustomType so
  custom type imports are registered directly on the context

Add behavioral tests for helper function templates (test/helpers/):
- genhelpers/main.go: CLI generator tool that reads ParamHelpersTemplate,
  MarshalFormHelperTemplate, and ParamTemplates registries, writes
  .gen.go files with goimports to prune unused imports
- 16 generated .gen.go files: helpers, marshal_form, plus style/bind for
  simple, simple_explode, form, form_explode, label, label_explode,
  and deepObject
- 7 test files (55 tests) covering marshalForm, primitiveToString,
  BindStringToObject, Date, escape/unescape, and roundtrip
  style-then-bind for all parameter styles

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mromaszewicz

experimental/internal/codegen/clientgen_test.go

@@ -35,10 +35,10 @@ func TestClientGenerator(t *testing.T) {
 	}
 
 	// Create param tracker
-	paramTracker := NewParamUsageTracker()
+	ctx := NewCodegenContext()
 
 	// Gather operations
-	ops, err := GatherOperations(doc, paramTracker, NewContentTypeMatcher(DefaultContentTypes()))
+	ops, err := GatherOperations(doc, ctx, NewContentTypeMatcher(DefaultContentTypes()))
 	require.NoError(t, err, "Failed to gather operations")
 	require.Len(t, ops, 4, "Expected 4 operations")
 
@@ -102,8 +102,8 @@ func TestClientGenerator_FormEncoded(t *testing.T) {
 		schemaIndex[s.Path.String()] = s
 	}
 
-	paramTracker := NewParamUsageTracker()
-	ops, err := GatherOperations(doc, paramTracker, contentTypeMatcher)
+	ctx := NewCodegenContext()
+	ops, err := GatherOperations(doc, ctx, contentTypeMatcher)
 	require.NoError(t, err, "Failed to gather operations")
 
 	// Verify we have an operation with a form-encoded body
@@ -131,7 +131,8 @@ func TestClientGenerator_FormEncoded(t *testing.T) {
 	require.Contains(t, clientCode, "marshalForm(body)")
 
 	// Verify we generate the form helper when needed
-	formHelper, err := generateFormHelper(ops)
+	ctx.NeedFormHelper(ops)
+	formHelper, err := generateMarshalFormHelper()
 	require.NoError(t, err, "Failed to generate form helper")
 	require.NotEmpty(t, formHelper, "Form helper should be generated when form-encoded bodies exist")
 	require.Contains(t, formHelper, "func marshalForm(")