Allow memory profiling free memory increases

[johnnyychiu]

Purpose

Fixes #45472.

Adds an opt-in VLLM_MEMORY_PROFILER_ALLOW_FREE_MEMORY_INCREASE=1 escape hatch for shared-GPU test setups where another process may release memory while vLLM profiles startup memory. Default behavior remains unchanged: vLLM raises if free GPU memory increases during profiling. When the env var is enabled, vLLM logs a warning and continues, making the risk explicit because KV cache memory can be overestimated.

Test Plan

• Add CPU-only unit coverage for the default raise path and opt-in warning path.
• Run Python bytecode compile on the touched files.
• Run whitespace check.

Test Result

• python.exe -m py_compile vllm\envs.py vllm\v1\worker\gpu_worker.py tests\v1\worker\test_worker_memory_snapshot.py passed.
• git diff --check passed.
• Not run: pytest ... because the available local Python runtime does not have pytest installed.

[github-actions]

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