fix(lambda): can't find entry file under ESM module system (#35797)

### Issue # (if applicable)

Closes #21630.

### Reason for this change

In the ESM module system, callsites returns filenames prefixed with 'file://'. This is not compatible with the NodeJS file utility functions such as fs.existsSync().

### Description of changes

Remove 'file://' prefix.

### Describe any new or updated permissions being added

No new IAM permissions are added.

### Description of how you validated changes

Unit tests. Integration test cannot be added as the integration test system uses CommonJS, it will be impossible to replicate the error which only happens under an ESM system. 

### Credits

This is mostly inspired by @okko for [his PR](#21802). He found the root cause and fix for this issue, but it didn't get merged due to a lack of integration tests.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Abogical

packages/aws-cdk-lib/aws-lambda-nodejs/lib/function.ts

@@ -292,5 +292,8 @@ function findDefiningFile(scope: Construct): string {
     throw new ValidationError('Cannot find defining file.', scope);
   }
 
-  return sites[definingIndex].getFileName();
+  // Fixes issue #21630.
+  // ESM modules return a 'file://' prefix to the filenames, this should be removed for
+  // compatibility with the NodeJS filesystem functions.
+  return sites[definingIndex].getFileName().replace(/^file:\/\//, '');
 }

packages/aws-cdk-lib/aws-lambda-nodejs/test/function.test.ts

@@ -37,6 +37,11 @@ let stack: Stack;
 beforeEach(() => {
   stack = new Stack();
   jest.clearAllMocks();
+  // pretend the calling file is in a fake file path
+  mockCallsites.mockImplementation(() => [
+    { getFunctionName: () => 'NodejsFunction' },
+    { getFileName: () => bockPath`function.test.ts` },
+  ]);
 });
 
 // We MUST use a fake file system here.
@@ -57,12 +62,6 @@ bockfs({
 });
 const bockPath = bockfs.workingDirectory('/home/project');
 
-// pretend the calling file is in a fake file path
-mockCallsites.mockImplementation(() => [
-  { getFunctionName: () => 'NodejsFunction' },
-  { getFileName: () => bockPath`function.test.ts` },
-]);
-
 afterAll(() => {
   bockfs.restore();
 });
@@ -231,6 +230,23 @@ test('throws when entry is not js/ts', () => {
   })).toThrow(/Only JavaScript or TypeScript entry files are supported/);
 });
 
+test('NodejsFunction with .js handler in an ESM package', () => {
+  // pretend the calling file is in a fake file path
+  // In ESM, callsites are prepended with 'file://'
+  mockCallsites.mockImplementation(() => [
+    { getFunctionName: () => 'NodejsFunction' },
+    { getFileName: () => `file://${bockPath`function.test.ts`}` },
+  ]);
+
+  // WHEN
+  new NodejsFunction(stack, 'handler2');
+
+  // THEN
+  expect(Bundling.bundle).toHaveBeenCalledWith(stack, expect.objectContaining({
+    entry: expect.stringContaining('function.test.handler2.js'), // Automatically finds .ts handler file
+  }));
+});
+
 test('accepts tsx', () => {
   const entry = bockPath`handler.tsx`;