#1475 expert_cache_segmented_test fails on HIP builds instead of skipping (--vram-elastic is CUDA-only)

Descrição

On HIP builds, `expert_cache_segmented_test` fails instead of skipping:

```
FAIL open ExpertCache: --vram-elastic (a segmented expert cache) is CUDA-only for now
```

`CMakeLists.txt` already registers it with `SKIP_RETURN_CODE 77`, so the fix is for the test to `return 77` when it is built without CUDA (or when `ExpertCache` reports the segmented mode unsupported), the same way other backend-specific tests skip. #918 lists it as one of the "known" HIP ctest failures, so every AMD ctest run currently shows one failure that isn't a bug.

Seen on an R9700 (gfx1201): ctest 90/95. The other failures were `ple_parity`, `expert_parity` and `pool_test` (they need model fixtures) and `hip_q2_zero` (filed separately).

**Environment:** Strata v0.1.40.3 (d5ea713), Linux (Ubuntu 24.04), AMD Radeon AI PRO R9700 (gfx1201, 32 GB), Ryzen 9 9900X, TheRock ROCm 7.10.0a20251120 (`gfx120X-all` wheels, setup's default), built by `./setup.sh --backend hip`, then `cmake -DSTRATA_BUILD_TESTS=ON` + `ctest --test-dir build-hip`.