[ET-VK][testing] Add ETVK_FORCE_NO_EXTENSIONS debug flag

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> [ET-VK][testing] Add ETVK_FORCE_NO_EXTENSIONS debug flag #17692
• [ET-VK][qconv] Add software fallback for dotPacked4x8AccSatEXT in q8ta_ shaders #17691
• [ET-VK][qconv] Read weight buffer as int in pack_q8_conv2d_weights shader #17690

Add a build config flag etvk.force_no_extensions that causes all
Vulkan extension support queries in Adapter.h to return false. This
makes it easy to test fallback code paths for devices that lack
optional extensions (e.g. 8-bit storage buffers, float16, dot product)
on a development host that would otherwise report support for them.

Enable with: --config etvk.force_no_extensions=1

Differential Revision: D94314257

[pytorch-bot]

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❌ 4 New Failures

As of commit 112f081 with merge base 63f9724 ():

NEW FAILURES - The following jobs have failed:

• Propose to merge ghstack orig PRs to main / Try to create a PR with ghstack /orig branch (gh)
  Process completed with exit code 1.
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 69696e009a13fe9a7cdd8c3cc2d9be4002e8b6d601b691b5a31898bbcd68d614 /exec failed with exit code 139
• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 1509a47eeb8c42a95ff8550d31202ff2e9119397fe24075dc0e057f015cf7b1a /exec failed with exit code 1
• Test CUDA Builds / unittest-cuda / linux-job (gh)
  RuntimeError: Command docker exec -t 18afc8828b1b3ae27118a19fa18aabb040bac1c87babe490d00982047ab48b0c /exec failed with exit code 1

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