server-stream: follow-up on SSE Replay Buffer (#23226) (#25047)

• server-stream : pimpl

• server-stream: prefix free functions with server_stream_

address review from ggerganov: scope the public stream functions under the server_stream_ prefix, matching server_stream_session_manager_start/stop.

• server-stream: guard session and manager state with the mutex

address review from ggerganov: make done, completed_ts and the GC running flag plain members under their mutex and set the condvar predicates under the lock. keep cancelled atomic for the lock-free should_stop poll.

• server-stream: trim comments to the non-obvious

address review from ggerganov: drop comments that restate the code, keep the concurrency, lifetime and ordering rationale. de-stale a few comments left by the pimpl: g_stream_sessions is now internal and the /v1/streams listing is gone.

• server-stream: update dev docs for the pimpl and prefix

reflect server_stream_session_manager_start/stop and the server_stream_ prefix, note the manager is now a file-static singleton hidden in the .cpp

• server-stream: move stream traces to debug level

keep the bring-up traces for diagnostics but off the default log: skip drain, draining, drain ended, DELETE evict, attach_pipe, and the router stream resume proxy.

• server-stream: align router stream resume proxy trace with upstream

the child-side bring-up traces are already SRV_TRC on master, move the router stream resume proxy trace to the same level.

• server-stream: move stream_read_status enum to the cpp

it is only used by the hidden session and consumer types, so it belongs with them behind the pimpl boundary, not on the public header surface.

Co-authored-by: Georgi Gerganov [email protected]