fix(agents-md): use require.resolve() to get the installed next version (#89166)

### Why?

When the package.json has Next.js version with a tag instead of a
specific version, e.g., `"next": "canary"`, it received an error:

```
Failed to pull docs: Could not find documentation for Next.js vcanary. This version may not exist on GitHub yet.
```

### How?

Modified logic to get version to use `require.resolve()`.

#### Before


https://github.com/user-attachments/assets/487d0404-ad81-4c5c-a341-cab9a0e03020

#### After


https://github.com/user-attachments/assets/4153873f-681f-4c46-8ee0-ee7913749b7b

Author: devjiwonchoi

packages/next-codemod/lib/__tests__/agents-md-e2e.test.js

@@ -5,6 +5,7 @@ const fs = require('fs')
 const path = require('path')
 const os = require('os')
 const { runAgentsMd } = require('../../bin/agents-md')
+const { getNextjsVersion } = require('../../lib/agents-md')
 
 /**
  * TRUE E2E TESTS
@@ -291,4 +292,35 @@ This is my project documentation.
       process.chdir(originalCwd)
     }
   }, 30000) // Increase timeout for git clone
+
+  describe('getNextjsVersion', () => {
+    const fixturesDir = path.join(__dirname, 'fixtures/agents-md')
+
+    it('returns the installed Next.js version from node_modules', () => {
+      const fixture = path.join(fixturesDir, 'next-specific-version')
+      const result = getNextjsVersion(fixture)
+
+      expect(result.version).toBe('15.4.0')
+      expect(result.error).toBeUndefined()
+    })
+
+    it('returns actual installed version, not the tag from package.json', () => {
+      // package.json has "next": "latest", but node_modules has version "16.0.0"
+      const fixture = path.join(fixturesDir, 'next-tag')
+      const result = getNextjsVersion(fixture)
+
+      // Should return the actual installed version, not "latest"
+      expect(result.version).toBe('16.0.0')
+      expect(result.error).toBeUndefined()
+    })
+
+    it('returns error when Next.js is not installed', () => {
+      // Use a directory where next is not installed
+      const nonNextDir = '/tmp'
+      const result = getNextjsVersion(nonNextDir)
+
+      expect(result.version).toBeNull()
+      expect(result.error).toBe('Next.js is not installed in this project.')
+    })
+  })
 })

packages/next-codemod/lib/__tests__/fixtures/agents-md/.gitignore

@@ -0,0 +1,2 @@
+# Allow node_modules in test fixtures
+!node_modules/

packages/next-codemod/lib/__tests__/fixtures/agents-md/next-specific-version/node_modules/next/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "next-specific-version",
+  "dependencies": {
+    "next": "15.4.0"
+  }
+}

packages/next-codemod/lib/__tests__/fixtures/agents-md/next-specific-version/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "next-tag",
+  "dependencies": {
+    "next": "latest"
+  }
+}

packages/next-codemod/lib/__tests__/fixtures/agents-md/next-tag/node_modules/next/package.json

@@ -16,27 +16,11 @@ interface NextjsVersionResult {
 }
 
 export function getNextjsVersion(cwd: string): NextjsVersionResult {
-  const packageJsonPath = path.join(cwd, 'package.json')
-
-  if (!fs.existsSync(packageJsonPath)) {
-    return {
-      version: null,
-      error: 'No package.json found in the current directory',
-    }
-  }
-
   try {
-    const packageJson = JSON.parse(fs.readFileSync(packageJsonPath, 'utf-8'))
-    const dependencies = packageJson.dependencies || {}
-    const devDependencies = packageJson.devDependencies || {}
-
-    const nextVersion = dependencies.next || devDependencies.next
-
-    if (nextVersion) {
-      const cleanVersion = nextVersion.replace(/^[\^~>=<]+/, '')
-      return { version: cleanVersion }
-    }
-
+    const nextPkgPath = require.resolve('next/package.json', { paths: [cwd] })
+    const pkg = JSON.parse(fs.readFileSync(nextPkgPath, 'utf-8'))
+    return { version: pkg.version }
+  } catch {
     // Not found at root - check for monorepo workspace
     const workspace = detectWorkspace(cwd)
     if (workspace.isMonorepo && workspace.packages.length > 0) {
@@ -56,11 +40,6 @@ export function getNextjsVersion(cwd: string): NextjsVersionResult {
       version: null,
       error: 'Next.js is not installed in this project.',
     }
-  } catch (err) {
-    return {
-      version: null,
-      error: `Failed to parse package.json: ${err instanceof Error ? err.message : String(err)}`,
-    }
   }
 }
 
@@ -525,18 +504,16 @@ function findNextjsInWorkspace(cwd: string, patterns: string[]): string | null {
   const versions: string[] = []
 
   for (const pkgPath of packagePaths) {
-    const packageJsonPath = path.join(pkgPath, 'package.json')
-    if (!fs.existsSync(packageJsonPath)) continue
-
     try {
-      const content = fs.readFileSync(packageJsonPath, 'utf-8')
-      const pkg = JSON.parse(content)
-      const nextVersion = pkg.dependencies?.next || pkg.devDependencies?.next
-      if (nextVersion) {
-        versions.push(nextVersion.replace(/^[\^~>=<]+/, ''))
+      const nextPkgPath = require.resolve('next/package.json', {
+        paths: [pkgPath],
+      })
+      const pkg = JSON.parse(fs.readFileSync(nextPkgPath, 'utf-8'))
+      if (pkg.version) {
+        versions.push(pkg.version)
       }
     } catch {
-      // Skip invalid package.json
+      // Next.js not installed in this package
     }
   }