[None][refactor] BREAKING: Migrate LlmRequest fields to Python - #19484

PR #19484 is not fully approved (reviewDecision: `REVIEW_REQUIRED`). Outstanding review requests: user `Tabrizian`, user `pcastonguay`, user `SimengLiu-nv`, user `schetlur-nv`, user `yizhang-nv`, user `allisonlim-nv`, team `NVIDIA/trt-llm-disagg-devs`, team `NVIDIA/trt-llm-kv-cache-manager-devs`. GitHub review requirements, CODEOWNERS, or a blocking review may require additional approvals. Unresolved review conversations are evaluated separately.