test: add ExpectDownload regression test for mobile emulation context

[AbhinavMir]

Summary

Adds a regression test, TestDownloadInMobileContext, that exercises Page.ExpectDownload under an iPad Mini emulation context (IsMobile: true, HasTouch: true). The test confirms that the page-level "download" channel event still fires correctly under mobile emulation and that Download.URL() / Download.SuggestedFilename() return the expected values.

Refs #579.

Notes on #579

I was unable to reproduce a real bug from the report. After investigating, the snippet in the issue clicks <a href="…some.jpg"> against a server that does not send Content-Disposition: attachment and the anchor has no download attribute, so the browser navigates to the resource instead of triggering a download — ExpectDownload then correctly times out. With the same iPad Mini / IsMobile context plus a properly configured download (this PR's test), ExpectDownload resolves as expected.

I'm opening this as a draft so a maintainer can decide whether:

1. This regression coverage is worth merging on its own, and/or
2. The issue should be closed with a comment pointing the reporter at the download attribute / Content-Disposition: attachment requirement.

Test plan

• go test ./tests -run TestDownloadInMobileContext -v — passes
• go test ./tests -run "TestDownloadBasic|TestDownloadCancel|TestDownloadInMobileContext" -v — all pass
• go vet ./... clean, gofmt clean