feat: Internal session id for offloading

[karen-sy]

Purpose

Lightweight propagation of session id to vLLM offloading (Mooncake, OffloadingConnector) constructs.
This PR is stacked on #48048 and depends on its typed request-level session_id plumbing.

Context:

• DEP/RFC: [RFC][Feature]: First-class session id for conversation aware routing #48049
• Base session ID plumbing PR: feat(frontend): session id plumbing into requests #48048
• Related motivation: sglang#29173 shows the kind of future cache/offload policy this metadata can support

Changes:

• Propagate session identity into Mooncake Store metadata:
  Request.session_id -> RequestTracker.session_id -> ReqMeta.session_id
• Preserve compatibility with the prior experimental path by falling back to sampling_params.extra_args["session_id"] in the Mooncake Store scheduler when typed Request.session_id is absent.
• Propagate typed session identity into the generic offloading connector:
  Request.session_id -> ReqContext.session_id

This is metadata plumbing only. It does not change routing, scheduling, eviction, retention, offload placement, or Mooncake Store behavior.

Test Plan

Passes added unit tests.

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @karen-sy.

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