### Description
Fixes#9945
The `getExtension` function (rewritten for performance in #9816) scans
the URI right-to-left, encountering dots in query parameter values (e.g.
hostnames like `app.example.com`, filenames like `en.vtt`) before
finding the `?` delimiter. This causes incorrect extension detection,
breaking MIME type resolution for URLs with signed parameters or other
query strings containing dots.
**Fix:** Split the single right-to-left scan into two passes:
1. **Left-to-right:** find the first `?` or `#` to establish the path
boundary
2. **Right-to-left:** scan backward within the path portion only,
looking for `.` or `/`
This preserves the lightweight character-scanning approach from #9816
without reintroducing the `goog.Uri` dependency.
**New test cases added:**
- Dots in query parameter values
(`?host=app.example.com&signature=abc.def`)
- Signed URLs with encoded paths and filenames in query strings
- Hostname-like values in query parameters (`?origin=cdn.example.co.uk`)
- No path extension with dotted query values (`/stream?file=video.mp4` →
`""`)
- Dots in fragment identifiers (`#t=1.5`)
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Marc-Antoine
Álvaro Velad Galván
Wojciech Tyczyński
Rodolphe Breton