Increase coverage (#1997)

## Launch Checklist

This PR increases coverage by adding unit tests to lib folde, replace
the skipped end to end placeholder with actual tests and adds more end
to end tests.

This was mostly done by AI (Claude opus 4.8) and I reviewed it and
requested changes where needed.

 - [x] Briefly describe the changes in this PR.
 - [x] Link to related issues.
- [x] Include before/after visuals or gifs if this PR includes visual
changes.
 - [x] Write tests for all new functionality.
 - [x] Add an entry to `CHANGELOG.md` under the `## main` section.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

e2e/modal-driver.ts

@@ -34,5 +34,45 @@ export class ModalDriver {
     close: async (key: string) => {
       await this.helper.when.click(key + ".close-modal");
     },
+
+    /**
+     * Adds a source of the given type from the sources modal, keeping whatever
+     * defaults that type's editor prefills.
+     */
+    addSource: async (sourceId: string, sourceType: string) => {
+      const { when } = this.helper;
+      await when.setValue("modal:sources.add.source_id", sourceId);
+      await when.select("modal:sources.add.source_type", sourceType);
+      await when.click("modal:sources.add.add_source");
+      await when.wait(200);
+    },
+
+    /** Adds one of the predefined public sources listed in the sources modal. */
+    addPublicSource: async (index = 0) => {
+      await this.helper.get.element(".maputnik-public-source-select").nth(index).click();
+    },
+
+    deleteFirstActiveSource: async () => {
+      await this.helper.get.element(".maputnik-active-source-type-editor-header-delete").first().click();
+    },
+
+    /** Fills one number box of a coordinate pair in the image/video source editor. */
+    setCoordinateValue: async (index: number, value: string) => {
+      const input = this.helper.get
+        .elementByTestId("modal:sources")
+        .locator(".maputnik-array input")
+        .nth(index);
+      await input.fill(value);
+      await input.blur();
+    },
+
+    exportCreateHtml: async () => {
+      await this.helper.get.element(".maputnik-modal-export-buttons button").last().click();
+    },
+
+    exportSaveStyle: async () => {
+      await this.helper.stubSaveFilePicker();
+      await this.helper.get.element(".maputnik-modal-export-buttons button").first().click();
+    },
   };
 }