[None][feat] Support final-boundary snapshot reuse for V2 Mamba

[VALLIS-NERIA]

@coderabbitai summary

Description

This PR adds optional final-boundary snapshot reuse for the V2 Mamba cache manager introduced by the V2 Mamba core PR.

The changes:

• Add the mamba_save_last_snapshot LLM option and propagate the stable reusable prompt boundary through executor requests without changing existing positional request APIs.
• Derive the stable boundary only when save-last mode is enabled, including requests pre-tokenized by serve.Router; preserve the forwarded prompt's special-token semantics and only accept a true token-ID prefix.
• Treat stable-template rendering as a best-effort optimization so unsupported templates do not reject otherwise valid chat requests.
• Reserve one final snapshot slot per concurrent request and save the last stable Mamba state independently of periodic snapshots.
• Guard the C++ memory estimator when mamba_state_cache_interval=0.
• Restrict save-last mode to the V2 Python manager and reject unsupported C++/mixed, fallback, and current disaggregated routes explicitly.
• Update telemetry, API-stability references, the golden LLM-args manifest, and focused unit tests.

This PR depends on the V2 Mamba core PR, #16471. It is independent of the V2 Mamba disaggregated-serving follow-up. Because an upstream PR cannot use a fork-only branch as its base, this draft temporarily shows the prerequisite changes until they merge and the branch is rebased.

Test Coverage

• Focused save-last, request-boundary, router, and request-priority tests (49 passed)
• C++ interval-zero estimator test (1 passed)
• Final request-priority, serving-boundary, and API-stability suite (79 passed)
• python3 scripts/generate_llm_args_golden_manifest.py
• pre-commit run --from-ref origin/main --to-ref HEAD

PR Checklist

• Please check this after reviewing the above items as appropriate for this PR.

[VALLIS-NERIA]

/bot run

[tensorrt-cicd]

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PR_Github #59618 [ run ] completed with state SUCCESS. Commit: 564b8fb
/LLM/main/L0_MergeRequest_PR pipeline #48055 completed with status: 'FAILURE'

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[VALLIS-NERIA]

/bot run

[tensorrt-cicd]

PR_Github #59670 [ run ] triggered by Bot. Commit: f2cb294 Link to invocation

[tensorrt-cicd]

PR_Github #59670 [ run ] completed with state FAILURE. Commit: f2cb294
/LLM/main/L0_MergeRequest_PR pipeline #48103 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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[VALLIS-NERIA]

Reopened as a stacked draft at VALLIS-NERIA#10 so it can target the V2 Mamba core branch and show only its single squashed commit. Closing this upstream draft as superseded.