WB-1804 Use h2 in ModalHeader (#2369)

## Summary:
Per [this conversation](https://khanacademy.slack.com/archives/C8Z9DSKC7/p1732258953822829), we want to update the headers in modals to use `h2` instead of the default `h3` that comes with `HeadingMedium`s

Issue: WB-1804

(Related: AX-340)

## Test plan:

- [x] Run unit and snapshot tests with `yarn test` and` yarn jest -u`
- [x] Verify in Storybook that `OnePaneDialog` and `ModalHeader` have `h2` titles instead of `h3`
![image](https://github.com/user-attachments/assets/703b69a2-bcf5-4176-afdb-84b5f6a9ab9f)

Author: Chalayyy

Reviewers: jandrade, Chalayyy

Required Reviewers: 

Approved By: jandrade

Checks: ✅ Chromatic - Get results on regular PRs (ubuntu-latest, 20.x), ✅ Lint / Lint (ubuntu-latest, 20.x), ✅ Test / Test (ubuntu-latest, 20.x, 2/2), ✅ Test / Test (ubuntu-latest, 20.x, 1/2), ✅ Check build sizes (ubuntu-latest, 20.x), ✅ Chromatic - Build on regular PRs / chromatic (ubuntu-latest, 20.x), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ⏭️  Chromatic - Skip on Release PR (changesets), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ gerald, ✅ Prime node_modules cache for primary configuration (ubuntu-latest, 20.x), ✅ Chromatic - Get results on regular PRs (ubuntu-latest, 20.x), ✅ Test / Test (ubuntu-latest, 20.x, 2/2), ✅ Test / Test (ubuntu-latest, 20.x, 1/2), ✅ Lint / Lint (ubuntu-latest, 20.x), ✅ Check build sizes (ubuntu-latest, 20.x), ✅ Chromatic - Build on regular PRs / chromatic (ubuntu-latest, 20.x), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ⏭️  Chromatic - Skip on Release PR (changesets), ✅ Prime node_modules cache for primary configuration (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ⏭️  dependabot, ✅ gerald

Pull Request URL: #2369

.changeset/gentle-moons-prove.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/wonder-blocks-modal": patch
+---
+
+Use h2 tag in modal header instead of h3

packages/wonder-blocks-modal/src/components/__tests__/modal-header.test.tsx

@@ -25,6 +25,16 @@ describe("ModalHeader", () => {
         expect(screen.getByText("Title")).toBeInTheDocument();
     });
 
+    test("uses an h2 tag for the title", () => {
+        // Arrange, Act
+        render(<ModalHeader title="Title" titleId="modal-title" />);
+
+        // Assert
+        expect(
+            screen.getByRole("heading", {level: 2, name: "Title"}),
+        ).toBeInTheDocument();
+    });
+
     test("using only `breadcrumbs` should render the header", () => {
         // Arrange, Act
         render(

packages/wonder-blocks-modal/src/components/modal-header.tsx

@@ -124,6 +124,7 @@ export default function ModalHeader(props: Props) {
                 <View style={styles.breadcrumbs}>{breadcrumbs}</View>
             )}
             <HeadingMedium
+                tag="h2"
                 style={styles.title}
                 id={titleId}
                 testId={testId && `${testId}-title`}