[ts-next-plugin] test: add metadata warn no type test (#78505)

### Why?

This PR initiates the migration of manual testing of the Next.js TypeScript plugin to an automated language service plugin test. It replaced the existing metadata `missing-type-warning` fixture with the `warn-no-type` test (naming for consistency, such as future `error-on-client`).

### Testing Plan

This PR adds the tests and fixtures for the metadata plugin to **warn when no type is provided** for metadata config exports (`metadata` and `generateMetadata`).

1. The fixtures are tested into two categories: 1) "**has type**": expects no warning, and 2) "**no type**": expects a warning.
2. For "has type", the types are from `next` or custom.
3. Each section consists of inline exports and separate exports (`export { ... }`).
4. For the fixtures of `generateMetadata`, it is divided into a function, an arrow function, and a function expression divided with sync and async.

<details><summary>Structure Details</summary>
<p>

#### `metadata`

```
.
├── has-type/
│   ├── export-inline/
│   │   ├── from-next
│   │   └── from-others
│   └── export-separate/
│       ├── from-next
│       └── from-others
└── no-type/
    ├── export-inline
    └── export-separate
```

#### `gerenateMetadata`

```
.
├── has-type/
│   ├── export-inline/
│   │   ├── from-next/
│   │   │   ├── async/
│   │   │   │   ├── arrow-function
│   │   │   │   ├── function-declaration
│   │   │   │   └── function-expression
│   │   │   └── sync/
│   │   │       ├── arrow-function
│   │   │       ├── function-declaration
│   │   │       └── function-expression
│   │   └── from-others/
│   │       ├── async/
│   │       │   └── ...
│   │       └── sync/
│   │           └── ...
│   └── export-separate/
│       ├── from-next/
│       │   ├── async/
│       │   │   └── ...
│       │   └── sync/
│       │       └── ...
│       └── from-others/
│           ├── async/
│           │   └── ...
│           └── sync/
│               └── ...
└── no-type/
    ├── export-inline/
    │   ├── async/
    │   │   └── ...
    │   └── sync/
    │       └── ...
    └── export-separate/
        ├── async/
        │   └── ...
        └── sync/
            └── ...
```

</p>
</details> 

### Follow Up

The metadata currently has one more feature: an error when using metadata exports within Client Components.

Author: devjiwonchoi

packages/next/src/server/typescript/rules/metadata.ts

@@ -95,7 +95,7 @@ const metadata = {
               file: source,
               category: ts.DiagnosticCategory.Warning,
               code: NEXT_TS_ERRORS.INVALID_METADATA_EXPORT,
-              messageText: `The "generateMetadata" export should have a return type of ${isAsync ? '"Promise<Metadata>"' : '"Metadata"'} from "next".`,
+              messageText: `The Next.js "generateMetadata" export should have a return type of ${isAsync ? '"Promise<Metadata>"' : '"Metadata"'} from "next".`,
               start: node.name.getStart(),
               length: node.name.getWidth(),
             },
@@ -135,7 +135,7 @@ const metadata = {
                   file: source,
                   category: ts.DiagnosticCategory.Warning,
                   code: NEXT_TS_ERRORS.INVALID_METADATA_EXPORT,
-                  messageText: `The "generateMetadata" export should have a return type of ${isAsync ? '"Promise<Metadata>"' : '"Metadata"'} from "next".`,
+                  messageText: `The Next.js "generateMetadata" export should have a return type of ${isAsync ? '"Promise<Metadata>"' : '"Metadata"'} from "next".`,
                   start: declaration.name.getStart(),
                   length: declaration.name.getWidth(),
                 },

test/development/typescript-plugin/.gitignore

@@ -0,0 +1 @@
+!tsconfig.json

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-async-arrow-function/layout.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Layout({ children }: { children: React.ReactNode }) {
+  return <>{children}</>
+}
+
+export const generateMetadata = async (): Promise<Metadata> => {
+  return {
+    title: 'Generate Metadata',
+  }
+}

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-async-arrow-function/page.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Page() {
+  return <p>hello world</p>
+}
+
+export const generateMetadata = async (): Promise<Metadata> => {
+  return {
+    title: 'Generate Metadata',
+  }
+}

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-async-function-expression/layout.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Layout({ children }: { children: React.ReactNode }) {
+  return <>{children}</>
+}
+
+export const generateMetadata = async function (): Promise<Metadata> {
+  return {
+    title: 'Generate Metadata',
+  }
+}

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-async-function-expression/page.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Page() {
+  return <p>hello world</p>
+}
+
+export const generateMetadata = async function (): Promise<Metadata> {
+  return {
+    title: 'Generate Metadata',
+  }
+}

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-async-function/layout.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Layout({ children }: { children: React.ReactNode }) {
+  return <>{children}</>
+}
+
+export async function generateMetadata(): Promise<Metadata> {
+  return {
+    title: 'Generate Metadata',
+  }
+}

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-async-function/page.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Page() {
+  return <p>hello world</p>
+}
+
+export async function generateMetadata(): Promise<Metadata> {
+  return {
+    title: 'Generate Metadata',
+  }
+}

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-sync-arrow-function/layout.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Layout({ children }: { children: React.ReactNode }) {
+  return <>{children}</>
+}
+
+export const generateMetadata = (): Metadata => {
+  return {
+    title: 'Generate Metadata',
+  }
+}

test/development/typescript-plugin/metadata/app/warn-no-type/generate-metadata/has-type/export-inline-from-next-sync-arrow-function/page.tsx

@@ -0,0 +1,11 @@
+import type { Metadata } from 'next'
+
+export default function Page() {
+  return <p>hello world</p>
+}
+
+export const generateMetadata = (): Metadata => {
+  return {
+    title: 'Generate Metadata',
+  }
+}