docs: add api handler service usage

docs/reference/server-adapters/nestjs.mdx

@@ -66,6 +66,97 @@ export class MyController {
 
 You can still use the regular Prisma service by injecting as usual.
 
+###  Using the API handler service to automate request handling
+
+This module also provides an `ApiHandlerService` that can be used to automate the request handling using API handlers. This is useful if you want to handle requests in a more structured way, such as using a controller or middleware. Before using it, you should ensure `ENHANCED_PRISMA` is registered in the module. For example:
+
+```ts
+import { ZenStackModule, ApiHandlerService } from '@zenstackhq/server/nestjs';
+import { enhance } from '@zenstackhq/runtime';
+import { PrismaService } from './prisma.service';
+
+@Module({
+  imports: [
+    // Register the ZenStack module
+    // The module exports the ENHANCED_PRISMA token and could be used in the ApiHandlerService
+    ZenStackModule.registerAsync({
+      useFactory: (prisma: PrismaService) => {
+        return {
+          getEnhancedPrisma: () => enhance(prisma, { user: ... }),
+        };
+      },
+      inject: [PrismaService],
+      extraProviders: [PrismaService],
+    }),
+  ],
+  providers: [ApiHandlerService]
+})
+export class AppModule {}
+```
+
+Then, you can inject the `ApiHandlerService` into your code and use it to handle requests. The service provides a method `handleRequest` to handle request using API handler automatically.
+
+RPC API Handler:
+
+```ts
+import { Controller, Get, Post } from '@nestjs/common';
+import { ApiHandlerService } from '@zenstackhq/server/nestjs';
+
+@Controller('post')
+export class PostController {
+  constructor(private readonly apiHandlerService: ApiHandlerService) {}
+
+  // should align with the route generated by API handler
+  @Get('findMany')
+  async findMany() {
+    return this.apiHandlerService.handleRequest()
+  }
+
+  @Post('create')
+  async create() {
+    return this.apiHandlerService.handleRequest()
+  }
+}
+```
+
+RESTful API Handler:
+
+```ts
+import { Controller, Get, Post } from '@nestjs/common';
+import { ApiHandlerService } from '@zenstackhq/server/nestjs';
+import RESTApiHandler from '@zenstackhq/server/api/rest';
+
+const ENDPOINT = 'http://localhost';
+
+@Controller('post')
+export class PostController {
+  constructor(private readonly apiHandlerService: ApiHandlerService) {}
+
+  // should align with the route generated by API handler
+  @Get()
+  async list() {
+    return this.apiHandlerService.handleRequest(
+      {
+        handler: RESTApiHandler({
+          endpoint: ENDPOINT,
+        }),
+      }
+    )
+  }
+
+  @Post()
+  async create() {
+    return this.apiHandlerService.handleRequest(
+      {
+        handler: RESTApiHandler({
+          endpoint: ENDPOINT,
+        }),
+      }
+    )
+  }
+}
+```
+
 ### API reference
 
 #### `ZenStackModule.registerAsync`
@@ -121,3 +212,57 @@ interface ZenStackModuleOptions {
   getEnhancedPrisma: (model?: string | symbol) => unknown;
 }
 ```
+
+#### `ApiHandlerService.handleRequest`
+
+##### Signature
+
+```ts
+handleRequest(options?: ApiHandlerOptions): Promise<unknown>;
+```
+
+##### Parameter `options`
+
+```ts
+interface ApiHandlerOptions {
+  /**
+   * Logger settings
+   */
+  logger?: LoggerConfig;
+
+  /**
+   * Model metadata. By default loaded from the `node_module/.zenstack/model-meta`
+   * module. You can pass it in explicitly if you configured ZenStack to output to
+   * a different location.
+   */
+  modelMeta?: ModelMeta;
+
+  /**
+   * Zod schemas for validating request input. Pass `true` to load from standard location
+   * (need to enable `@core/zod` plugin in schema.zmodel) or omit to disable input validation.
+   */
+  zodSchemas?: ZodSchemas | boolean;
+
+  /**
+   * Api request handler function. Can be created using `@zenstackhq/server/api/rest` or `@zenstackhq/server/api/rpc` factory functions.
+   * Defaults to RPC-style API handler.
+   */
+  handler?: HandleRequestFn;
+
+  /**
+   * The base URL for the API handler. This is used to determine the base path for the API requests.
+   * If you are using the ApiHandlerService in a route with a prefix, you should set this to the prefix.
+   * 
+   * e.g.  
+   * without baseUrl(API handler default route):
+   * - RPC API handler: [model]/findMany
+   * - RESTful API handler: /:type
+   * 
+   * with baseUrl(/api/crud):
+   * - RPC API handler: /api/crud/[model]/findMany
+   * - RESTful API handler: /api/crud/:type
+   */
+  baseUrl?: string;
+}
+```
+