[cross_file] [web] Separate "Save As" implementation details from XFile web class

[raju-muliyashiya]

fixes Issue #91400

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List which issues are fixed by this PR. You must list at least one issue.

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• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request refactors the 'Save As' functionality for the web implementation of XFile. The implementation details are moved from the XFile class into a new saveFileAs helper function in web_helpers.dart, which improves separation of concerns. The testing mechanism is also updated to use a global override instead of passing an override object.

My review includes suggestions to improve the new helper function for efficiency and clarity, and to ensure test hygiene by properly cleaning up the global test override.

[gemini-code-assist]

It's a good practice to clean up global state modifications within tests to prevent them from affecting other tests. Using addTearDown ensures that the override is reset even if the test fails.

        addTearDown(() => helpers.anchorElementOverride = null);
        helpers.anchorElementOverride = (_, __) => mockAnchor;

[gemini-code-assist]

This function can be improved for clarity and efficiency. The path parameter is unused and can be removed. Additionally, clearing the target element's children can be done more efficiently and expressively using replaceChildren. This also removes the need for the addElementToContainerAndClick helper for this use case. Remember to update the call site in XFile.saveTo.

/// Saves the given [XFile] to user's device ("Save As" dialog).
Future<void> saveFileAs(XFile file) async {
  // Create container element.
  final Element target = ensureInitialized('__x_file_dom_element');

  // Create <a> element.
  final HTMLAnchorElement element =
      anchorElementOverride != null
          ? anchorElementOverride!(file.path, file.name)
          : createAnchorElement(file.path, file.name);

  // The new element replaces all the existing children of the container.
  target.replaceChildren(element);
  // Click the element to trigger the download.
  element.click();
}