[test] extent tests, coverage and harden responses

[herostrat]

In preparation of new features I experiment with I decided to upgrade the tests

I think I found also found and fixed some inconsistencies:
Filesystem tile serving now validates the tile format and ensures the target path is a readable, regular file before serving. If not, it returns 404.
MBTiles serving also validates the tile format and returns 404 for unsupported formats.
The empty-chart scenario no longer yields false positives: resources may be missing, but any tile request returns 404.

The API serves individual tiles, not containers. When a tile is missing, unreadable, out of range, or in an unsupported format, the client should receive a clear “not found” response. This prevents accidental 200s for invalid paths or files and avoids leaking unintended filesystem content.

So I added integration tests for unsupported formats, unreadable tiles, invalid parameters, out-of-range zoom, and out-of-range tile coordinates.

Note: the new tests were primarily created with the help of AI.