Fix issue with duplicate images

[fatlewis]

Fixes #29079.

I've added a method on the Exporter class to track duplicate file names and append an incrementing number to the duplicates, e.g.:

"foo/bar/file.txt"
"foo/bar/file (1).txt"
"foo/bar/file (2).txt"

I've also added a test for this feature to the HTMLExport test suite, as that was the area in which the issue was discovered. While adding this test I found that the mock for createMessagesRequest wasn't resolving the correct value, which meant that tests were duplicating events, so I've corrected that (I'm a bit surprised this wasn't discovered before).

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation. (N/A)
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)