test: preserve default ROCm backend behavior

[Peter9606]

Summary

This PR adds guardrail tests for FlyDSL’s multi-backend defaults.

The tests ensure that:

• Python compile backend defaults still resolve to rocm
• Python device runtime defaults still resolve to rocm
• registering an additional Python backend does not change the default backend selection
• CMake defaults still select only the rocdl backend
• ROCm runtime wrappers are only included when rocdl is selected
• future CMake backend descriptors are included only when explicitly selected

Motivation

FlyDSL now has infrastructure for multiple hardware backends. As new backend stacks are added, community users who do not opt into those backends should see no behavior change: default imports, backend selection, runtime pairing, tests, and builds should continue to use the existing ROCm/ROCDL path.

This PR makes that invariant explicit without adding any vendor-specific backend. The CMake test uses a temporary dummy backend descriptor to verify that future backend descriptors remain opt-in.

Test plan

PYTHONPATH="$PWD/python:$PWD" python3 -m pytest \
  tests/unit/test_device_runtime.py \
  tests/unit/test_compile_backends.py \
  tests/unit/test_backend_cmake_defaults.py \
  -v --confcutdir=tests/unit

Result: 15 passed.

[coderfeli]

LGTM. Merged @Peter9606