[webview_flutter_android] Add headers to NavigationRequest to tell if it isDownloadRequest from onDownloadStart

[wuzhengu]

Add headers and isDownloadRequest to NavigationRequest，so we can easily tell if it was made to download an attachment from onDownloadStart.

List which issues are fixed by this PR. You must list at least one issue.
132738

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[gemini-code-assist]

Code Review

This pull request updates the webview_flutter_android and webview_flutter_platform_interface packages to include request headers in NavigationRequest and introduces an isDownloadRequest getter to identify attachment downloads. The Android implementation is updated to populate these headers during download events. Feedback suggests making the 'attachment' string comparison case-insensitive to comply with RFC 6266 and adjusting Dart formatting.

[gemini-code-assist]

The check for the attachment prefix should be case-insensitive, as HTTP header values are case-insensitive per RFC 6266. Also, there should be a space after the if keyword to follow standard Dart styling.

Suggested change

	if(!contentDisposition.startsWith('attachment')) {
	contentDisposition = 'attachment; $contentDisposition'.trim();
	}
	if (!contentDisposition.toLowerCase().startsWith('attachment')) {
	contentDisposition = 'attachment; $contentDisposition'.trim();
	}

[gemini-code-assist]

The isDownloadRequest getter should perform a case-insensitive check for the attachment disposition type to comply with RFC 6266.

Suggested change

	bool get isDownloadRequest => headers?['content-disposition']?.startsWith('attachment') ?? false;
	bool get isDownloadRequest => headers?['content-disposition']?.toLowerCase().startsWith('attachment') ?? false;