testHgiVulkan failure: Deferred command buffer submission breaks WaitForIdle, command buffer reuse, and garbage collection

[erikaharrison-adsk]

Steps to reproduce:

• Build and run testHgiVulkan on Windows, NVIDIA RTX A1000 6GB Laptop GPU

Notes:

• Seems introduced by 30ce89430c
• (with help from AI) The introduction of deferred command buffer submission means that vkQueueSubmit calls are now buffered rather than executed immediately, but is HgiVulkanDevice::WaitForIdle() still only called vkDeviceWaitIdle() — which returns instantly when no work has been submitted to the GPU. This leaves command buffers stuck in an "in-flight" state, preventing reuse and blocking the garbage collector from freeing GPU resources (since their inflight bits were never released).

Build command (likely most of this is not relevant):
python build_scripts\build_usd.py --jobs 4 --build-variant "debug" --openimageio --tests --vulkan --materialx --onetbb --build-args OpenImageIO,-DCMAKE_FIND_USE_SYSTEM_PACKAGE_REGISTRY=FALSE USD --build <build path> <install path>

Test command: ctest -C Debug -R testHgiVulkan -V

Test fail error message:

903: *** Running test: HgiVulkan
903:
903: Command buffers were not reused
903: Inflight bits invalid (2)
903: TestVulkanCommandQueue failed
903: HgiVulkan: Failed
903: Error: return code 1 doesn't match expected 0 (EXPECTED_RETURN_CODE).chdir: ...

[jesschimein]

Filed as internal issue #USD-11882

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