fix(distserve): free Prefill cache with scheduler session id

[gokay-ai]

Motivation

DistServe Proxy /v1/completions can leak Prefill scheduler sessions until the Prefill ZMQ engine dies.

After Prefill, the proxy set migration_request.remote_session_id from the OpenAI response id. That field is the client session_id (default -1), not the internal Prefill scheduler session. Decode then cache-frees -1, Prefill logs invalid free, and the preserve_cache sequence stays in scheduler.sessions / seq_manager. Chat has the same mismatch (chatcmpl-* is not an int).

Fixes #4967

Modification

• Prefill /v1/completions and /v1/chat/completions return cache_session_id (engine/scheduler session id) with the DistServe cache fields.
• Proxy uses that id for remote_session_id and PD shelf/unshelf. Missing or invalid owners are rejected instead of falling back to the public id.
• Unit tests cover the public-id miss, response plumbing, proxy migration/free, and no-fallback reject path. No DistServe cluster required.

Related: #4977 also plumbs a cache owner id as part of a broader DistServe change. This PR is scoped to the #4967 free-path mismatch only.

BC-breaking (Optional)

Proxy and Prefill servers should be upgraded together: DistServe proxy now requires cache_session_id from Prefill. Public OpenAI response id is unchanged.

Checklist

1. Ruff check on the touched files passed; focused pytest: 8 passed.
2. Unit tests added for the session-id mismatch and proxy plumbing.
3. N/A
4. N/A

[gokay-ai]

Standing down in favor of #4977 (RunningLeon), which already returns cache_session_id from Prefill and uses it for proxy migration/free (same root cause as #4967), plus related DistServe fixes. No need for a second overlapping PR.