pytest.raises(match=...) is case-sensitive by default. The error
message starts with "Neither" (capital N) but the regex used lowercase
"neither", causing CI failures on Linux.
When neither apptainer nor singularity is installed, the Singularity
backend silently defaults to "singularity" and fails with a cryptic
FileNotFoundError inside _start_instance(). Add a preflight check
that resolves the executable and verifies it responds, raising a
clear RuntimeError with install instructions on failure.
Closes#1511