test(spawn-bundle-fallback): shorter session names to stay under socket path limit
The PR's new tests generated session names like `bundle-fb1-ov5o` (15 chars).
Combined with macOS's tempdir under /var/folders/.../T/ (~52 chars) plus the
test root prefix and per-test "d-XXXXXX/", the resulting socket path tipped
over the 104-byte Unix-socket kernel limit by 1 byte.
Shorten the name format to ~6 chars (b<counter><rand3>); leaves enough margin
on /var/folders/... tempdirs.