feat(gen): add x-ogen-name extension for parameter naming

Add support for the x-ogen-name extension on OpenAPI parameters, allowing
users to customize Go struct field names independently of the OpenAPI
parameter name. This mirrors the existing x-ogen-name functionality for
schema types.

Changes:
- Add XOgenName field to openapi.Parameter struct
- Parse x-ogen-name extension in parse_parameter.go with Go identifier
  validation (must be valid and exported)
- Use XOgenName in gen_parameters.go for Go field naming
- Add positive and negative test cases
- Add integration test with generated code

Closes #1593

Author: lanej

_testdata/positive/param_naming_extensions.json

@@ -0,0 +1,58 @@
+{
+  "openapi": "3.0.3",
+  "info": {
+    "title": "Parameter Naming Extensions Test",
+    "version": "v0.1.0"
+  },
+  "paths": {
+    "/items/{itemid}": {
+      "get": {
+        "operationId": "getItem",
+        "parameters": [
+          {
+            "name": "itemid",
+            "in": "path",
+            "required": true,
+            "x-ogen-name": "ItemID",
+            "schema": {
+              "type": "string"
+            }
+          },
+          {
+            "name": "pickuptype",
+            "in": "query",
+            "x-ogen-name": "PickupType",
+            "schema": {
+              "type": "string"
+            }
+          },
+          {
+            "name": "x-custom-header",
+            "in": "header",
+            "x-ogen-name": "CustomHeader",
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Item info",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "id": {
+                      "type": "string"
+                    }
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}

gen/gen_parameters.go

@@ -174,9 +174,16 @@ func (g *Generator) generateParameter(ctx *genctx, opName string, p *openapi.Par
 		return nil, errors.Wrapf(err, "%q", p.Name)
 	}
 
-	paramName, err := pascalNonEmpty(p.Name)
-	if err != nil {
-		return nil, errors.Wrapf(err, "parameter name: %q", p.Name)
+	var paramName string
+	if p.XOgenName != "" {
+		// Use custom name from x-ogen-name extension.
+		paramName = p.XOgenName
+	} else {
+		var err error
+		paramName, err = pascalNonEmpty(p.Name)
+		if err != nil {
+			return nil, errors.Wrapf(err, "parameter name: %q", p.Name)
+		}
 	}
 
 	var tag ir.Tag

internal/integration/generate.go

@@ -40,6 +40,7 @@ package integration
 //
 //go:generate go run ../../cmd/ogen -v --clean -target test_enum_naming       ../../_testdata/positive/enum_naming.yml
 //go:generate go run ../../cmd/ogen -v --clean -target test_naming_extensions ../../_testdata/positive/naming_extensions.json
+//go:generate go run ../../cmd/ogen -v --clean -target test_param_naming_extensions ../../_testdata/positive/param_naming_extensions.json
 //go:generate go run ../../cmd/ogen -v --clean -target test_type_extension ../../_testdata/positive/type_extension.yml
 //go:generate go run ../../cmd/ogen -v --clean -target test_type_extension_name ../../_testdata/positive/type_extension_name.yml
 //go:generate go run ../../cmd/ogen -v --clean -target test_time_extension ../../_testdata/positive/time_extension.yml

internal/integration/param_naming_extensions_test.go

@@ -0,0 +1,35 @@
+package integration
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	api "github.com/ogen-go/ogen/internal/integration/test_param_naming_extensions"
+)
+
+func TestParamNamingExtensions(t *testing.T) {
+	// This test verifies that x-ogen-name extension works for parameters.
+	// The spec defines parameters with custom Go field names:
+	// - itemid -> ItemID
+	// - pickuptype -> PickupType
+	// - x-custom-header -> CustomHeader
+	a := require.New(t)
+
+	// Create params using the custom field names
+	params := api.GetItemParams{
+		ItemID: "test-item-id",
+	}
+
+	// Verify the field types exist with the custom names
+	a.Equal("test-item-id", params.ItemID)
+
+	// Test optional fields
+	params.PickupType.SetTo("delivery")
+	a.True(params.PickupType.IsSet())
+	a.Equal("delivery", params.PickupType.Value)
+
+	params.CustomHeader.SetTo("custom-value")
+	a.True(params.CustomHeader.IsSet())
+	a.Equal("custom-value", params.CustomHeader.Value)
+}