chore(prlint): ensure scope is lowercase (#33103)

PR linter now checks for lowercase scope. 

Valid: `fix(s3): blah`

Invalid: `fix(S3): blah`

### 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*

tools/@aws-cdk/prlint/lint.ts

@@ -410,7 +410,7 @@ function validateTitlePrefix(pr: GitHubPr): TestResult {
 }
 
 /**
- * Check that the PR title uses the typical convention for package names.
+ * Check that the PR title uses the typical convention for package names, and is lowercase.
  *
  * For example, "fix(s3)" is preferred over "fix(aws-s3)".
  */
@@ -427,7 +427,17 @@ function validateTitleScope(pr: GitHubPr): TestResult {
   if (m && !scopesExemptFromThisRule.includes(m[2])) {
     result.assessFailure(
       !!(m[2] && m[3]),
-      `The title of the pull request should omit 'aws-' from the name of modified packages. Use '${m[3]}' instead of '${m[2]}'.`,
+      `The title scope of the pull request should omit 'aws-' from the name of modified packages. Use '${m[3]}' instead of '${m[2]}'.`,
+    );
+  }
+
+  // Title scope is lowercase
+  const scopeRe = /^\w+\(([\w_-]+)\)?: /; // Isolate the scope
+  const scope = scopeRe.exec(pr.title);
+  if (scope && scope[1]) {
+    result.assessFailure(
+      scope[1] !== scope[1].toLocaleLowerCase(),
+      `The title scope of the pull request should be entirely in lowercase. Use '${scope[1].toLocaleLowerCase()}' instead.`,
     );
   }
   return result;

tools/@aws-cdk/prlint/test/lint.test.ts

@@ -142,7 +142,25 @@ describe('commit message format', () => {
       },
     };
     const prLinter = configureMock(issue, undefined);
-    await expect(legacyValidatePullRequestTarget(prLinter)).rejects.toThrow(/The title of the pull request should omit 'aws-' from the name of modified packages. Use 's3' instead of 'aws-s3'./);
+    await expect(legacyValidatePullRequestTarget(prLinter)).rejects.toThrow(/The title scope of the pull request should omit 'aws-' from the name of modified packages. Use 's3' instead of 'aws-s3'./);
+  });
+
+  test('invalid scope with capital letters', async () => {
+    const issue = {
+      number: 1,
+      title: 'fix(S3): some title',
+      body: '',
+      labels: [{ name: 'pr-linter/exempt-test' }, { name: 'pr-linter/exempt-integ-test' }],
+      user: {
+        login: 'author',
+      },
+    };
+    const prLinter = configureMock(issue, undefined);
+    await expect(prLinter.validatePullRequestTarget()).resolves.toEqual(expect.objectContaining({
+      requestChanges: expect.objectContaining({
+        failures: ["The title scope of the pull request should be entirely in lowercase. Use 's3' instead."],
+      }),
+    }));
   });
 
   test('valid scope with aws- in summary and body', async () => {