[None][feat] improve iteration stats reporting

[yizhang-nv]

Summary:

• Allow -1 as the unbounded sentinel for iter/request stats buffers.
• Preserve C++ semantics where 0 disables iter/request stats, while Python max_stats_len=0 keeps the legacy bounded-buffer trim behavior.
• Avoid overlapping iteration-stats drains in the OpenAI server metrics path.
• Track KV cache peak block stats by cache level and report primary/secondary peak free, used, and evictable blocks.
• Serialize the new peak free/evictable fields alongside the existing peak used fields, with C++ binding and serializer coverage.

Testing:

• python3 scripts/build_wheel.py --use_ccache --skip_building_wheel --linking_install_binary
• pytest -s tests/unittest/bindings/test_executor_bindings.py::test_executor_config
• pytest -s tests/unittest/executor/test_stats_serializer.py
• tests/unittest/llmapi/apps/test_disagg_perf_metrics_collector.py
• tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py

[yizhang-nv]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #51858 [ run ] triggered by Bot. Commit: 3ad52f0 Link to invocation

[tensorrt-cicd]

PR_Github #51858 [ run ] completed with state SUCCESS. Commit: 3ad52f0
/LLM/main/L0_MergeRequest_PR pipeline #41216 completed with status: 'FAILURE'

CI Report

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• Please check the failed tests and fix your PR
• If you cannot view the failures, ask the CI triggerer to share details
• Once fixed, request an NVIDIA team member to trigger CI again

CI Agent Failure Analysis

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[yizhang-nv]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #51982 [ run ] triggered by Bot. Commit: 1224a72 Link to invocation

[tensorrt-cicd]

PR_Github #51982 [ run ] completed with state FAILURE. Commit: 1224a72
/LLM/main/L0_MergeRequest_PR pipeline #41328 completed with status: 'FAILURE'

CI Report

⚠️ Action Required:

• Please check the failed tests and fix your PR
• If you cannot view the failures, ask the CI triggerer to share details
• Once fixed, request an NVIDIA team member to trigger CI again

CI Agent Failure Analysis

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[yizhang-nv]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #52251 [ run ] triggered by Bot. Commit: 9b55558 Link to invocation

[tensorrt-cicd]

PR_Github #52251 [ run ] completed with state SUCCESS. Commit: 9b55558
/LLM/main/L0_MergeRequest_PR pipeline #41565 completed with status: 'SUCCESS'

CI Report

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