Resolve the NXP profiler paths on use, not at import

[shoumikhin]

A small config file in the NXP test support code refuses to load on any machine that
does not have NXP's hardware tools installed. That matters because it is not only
used by the hardware tests. Shared test helpers import it, and the wheel ships those
helpers, so importing one of them from an installed wheel failed outright.

Two things caused it, both running the moment the file loads rather than when a value
is needed. It imports the eIQ Neutron SDK (the vendor toolkit that talks to the
hardware), which is not installed on an ordinary machine. It also builds a path from
shutil.which("nsys"), and which returns None when the profiler is missing, so
pathlib.Path then raises TypeError.

The four values that need the toolchain are now functions, resolved when something
asks for them:

nsys_path() the profiler binary
nsys_config_path() its ini file
nsys_firmware_path() the Neutron firmware, which needs the SDK
neutron_test_path() the built executor runner

PROJECT_DIR and OUTPUTS_DIR stay as they were, since neither needs the toolchain.

Both callers now call these instead of reading a constant. One of them only wanted to
know whether the profiler exists, not where it is, so that check asks shutil directly
and still tolerates a missing profiler rather than failing.

Nothing changes on a machine that has the toolchain: the same paths are computed
from the same inputs, with the same assertions, only later. On a machine without it,
the failure moves from import time to the first call that actually needs a path, and
says which piece is missing.

Test plan

Installed the wheel into a clean environment with no vendor toolkit and no profiler,
then imported the file. Before this change it failed with ModuleNotFoundError. After
it, the import succeeds and the project directory still resolves. Each of the four
functions still raises there, naming what is absent.

Confirmed no module level statement in config.py needs the toolchain any more, by
walking the parsed file and looking for one.

Note the shared helpers still do not import from a wheel, for a different reason
that this change does not touch: backends/nxp/backend/neutron_converter_manager.py
requires the SDK deliberately, in production code rather than in test support.

cc @robert-kalmar @JakeStevens @digantdesai @rascani

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