fix(tests): keep shadows cold before failover quiesce

[galletas1712]

Overview:

Cherry-pick of main PR #8258 into release/1.1.0 to stabilize the GMS fault-tolerance tests.

Details:

• Clean cherry-pick of main commit c69e19e8dd029d39d33a24dd42ffc3a111b124ae
• No release-only code changes
• No known cherry-pick dependencies
• Carries the same test changes as #8258:
  • keep shadow-a and shadow-b cold before their first quiesce in tests/gpu_memory_service/test_shadow_failover.py
  • remove repo-level NVML memory-accounting assertions from the GMS test flows
  • lower the vLLM / SGLang GMS harness memory fractions from 0.9 to 0.8

Where should the reviewer start?

• tests/gpu_memory_service/test_shadow_failover.py
• tests/gpu_memory_service/flow_assertions.py
• tests/gpu_memory_service/common/runtime.py

Validation:

• git cherry-pick --signoff c69e19e8dd029d39d33a24dd42ffc3a111b124ae
• Cherry-pick applied cleanly on top of origin/release/1.1.0
• Main PR #8258 passed isolated L4 pre-merge GMS validation before merge
• Release-branch CI pending on this PR

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to fix(tests): keep shadows cold before failover quiesce #8258
• Relates to nightly GMS shadow failover instability

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.