Add keepalives and safe recreation to session management

[merlimat]

Summary

• Sends periodic KeepAlive RPCs from a background thread so ephemeral state tied to a client session stays alive on the server.
• Teaches SessionManager to transparently recreate sessions that have been closed, while making on_session_closed race-safe so a late callback from an already-replaced session cannot drop a live replacement.
• Adds tests/sessions_test.py with deterministic fake stubs; the whole new suite runs in well under a second.

This is an alternative to #6 that addresses the review feedback from both Copilot and Codex.

Heartbeat cadence

_resolve_heartbeat_interval_ms() picks a cadence of roughly session_timeout_ms / 10, floored at 2s, and — crucially — capped strictly below session_timeout_ms. The cap is the fix for the small-timeout foot-gun flagged on #6: with max(session_timeout_ms // 10, 2_000) alone, any session_timeout_ms < 2000 makes the loop sleep past the timeout, so the local-expiry check fires before a single heartbeat is ever sent. The cadence is also injectable for tests, which is how sessions_test.py stays fast and deterministic without sleeping for wall-clock seconds.

Race fix in on_session_closed

Session.close() now notifies the manager before running the potentially slow close_session RPC, so replacement sessions created during the RPC don't race with the callback. And on_session_closed only evicts when the tracked entry is still the same object:

if self.sessions_by_shard.get(session.shard_id()) is session:
    del self.sessions_by_shard[session.shard_id()]

Without this, a late callback from an old session could evict a newer replacement that get_session() had installed during the close-RPC window, leaving a live session with a running heartbeat thread completely unmanaged.

Test plan

• pytest tests/sessions_test.py — 12 passed in ~0.35s, run 5x with no flakes
• pytest tests/compare_test.py tests/sessions_test.py — 13 passed
• python -c "import oxia" — no import regressions
• Integration run (client_test.py) against a real Oxia server — requires Docker locally, please run on CI