OpenVikingMemoryProvider only overrides on_session_end and inherits the
base-class no-op for on_session_switch. When the agent rotates session_id
(via /new, /branch, /reset, /resume, or context compression), the
provider's cached _session_id stays at the value initialize() captured.
All subsequent sync_turn writes then land in the already-closed old
session, and on_session_end tries to commit it a second time — the new
session never accumulates messages and never triggers memory extraction.
The fix mirrors the pattern Hindsight uses (#17508):
1. Wait for any in-flight sync thread to drain under the OLD _session_id
before we mutate it, otherwise the commit below races the last
message write.
2. Commit the old session if it accumulated turns — same extraction
semantics as on_session_end. Skip if empty (nothing to extract).
3. Drain in-flight prefetch from the old session and clear its cached
result so the new session doesn't see stale recall.
4. Rotate _session_id to the new value and reset _turn_count.
Commit failures are swallowed (logged at WARN) so a flaky server can't
strand the provider on the old session forever — same posture as the
existing on_session_end commit.
(cherry picked from commit a1e7185e8aea978e76163a288ac0cd5ee911290b)
813a4e3838