Record Windows world-writable scan telemetry (#40983)

## What changed

- Return the number of directories flagged by successful Windows sandbox world-writable scans.
- Record that count in the `codex.windows_sandbox.world_writable_scan_flagged_directories` histogram for startup and warning-triggered scans.
- Tag each measurement with a `success` or `error` result, using a count of zero when the scan fails.

GitOrigin-RevId: 076160f36b1d62fa737f4b2d0318774e6c44a4b1