feat: improve oneOf schema handling

This introduces an enhancement to better handle the oneOf schema in OpenAPI definitions.

- Added a new OpenAPI test fixture - oneOfMerge.yaml
- Updated logic in the codegen script where oneOf schemas are encountered. The properties of the schema are now deeply merged to accommodate different variations.
- Added a new test case to confirm the improved handling of oneOf schemas.

more details: #503 (comment)

fix: #495

--

Signed-off-by: kahiro okina <kahiro.okina@craftsman-software.com>

src/codegen/__fixtures__/oneOfMerge.yaml

@@ -0,0 +1,36 @@
+openapi: 3.0.3
+info:
+  title: OneOfMerge
+  version: 0.1.0
+paths:
+  /mix:
+    get:
+      description: stats
+      parameters:
+        - name: param1
+          in: query
+          schema:
+            oneOf:
+            - required:
+              - c
+              properties:
+                a:
+                  type: string
+            - required:
+              - b
+              properties:
+                b:
+                  type: string
+            required:
+            - d
+            properties:
+              c:
+                type: string
+              d:
+                enum:
+                - enum1
+                - enum2
+                type: string
+      responses:
+        "200":
+          description: ok

src/codegen/generate.ts

@@ -5,6 +5,7 @@ import { OpenAPIV3 } from "openapi-types";
 import * as cg from "./tscodegen";
 import generateServers, { defaultBaseUrl } from "./generateServers";
 import { Opts } from ".";
+import merge from "deepmerge";
 
 export const verbs = [
   "GET",
@@ -574,8 +575,14 @@ export default class ApiGenerator {
     }
 
     if (schema.oneOf) {
+      const clone = { ...schema };
+      delete clone.oneOf;
       // oneOf -> union
-      return this.getUnionType(schema.oneOf, schema.discriminator, onlyMode);
+      return this.getUnionType(
+        schema.oneOf.map((variant) => merge(variant as any, clone as any)),
+        schema.discriminator,
+        onlyMode,
+      );
     }
     if (schema.anyOf) {
       // anyOf -> union

src/codegen/index.test.ts

@@ -115,6 +115,13 @@ describe("generateSource", () => {
     expect(src).toContain("PathsFilterGetParameters0SchemaOneOf0");
   });
 
+  it("should merge properties within oneOf schema variations", async () => {
+    const src = await generate("/__fixtures__/oneOfMerge.yaml");
+    expect(src).toContain(
+      '{ param1?: { a?: string; c: string; d: "enum1" | "enum2"; } | { b: string; c?: string; d: "enum1" | "enum2"; }'
+    );
+  });
+
   it("should support parameters specified with content", async () => {
     const src = await generate("/__fixtures__/contentParams.json");
     expect(src).toContain(