fix(gateway): pass session_key (not session_id) to active-process check during prune

SessionStore.prune_old_entries was calling
self._has_active_processes_fn(entry.session_id) but the callback wired
up in gateway/run.py is process_registry.has_active_for_session, which
compares against session_key, not session_id. Every other caller in
session.py (_is_session_expired, _should_reset) already passes
session_key, so prune was the only outlier — and because session_id and
session_key live in different namespaces, the guard never fired.

Result in production: sessions with live background processes (queued
cron output, detached agents, long-running Bash) were pruned out of
_entries despite the docstring promising they'd be preserved. When the
process finished and tried to deliver output, the session_key to
session_id mapping was gone and the work was effectively orphaned.

Also update the existing test_prune_skips_entries_with_active_processes,
which was checking the wrong interface (its mock callback took session_id
so it agreed with the buggy implementation). The test now uses a
session_key-based mock, matching the production callback's real contract,
and a new regression guard test pins the behaviour.

Swallowed exceptions inside the prune loop now log at debug level instead
of silently disappearing.

gateway/session.py

@@ -926,12 +926,18 @@ class SessionStore:
                     continue
                 # Never prune sessions with an active background process
                 # attached — the user may still be waiting on output.
+                # The callback is keyed by session_key (see process_registry.
+                # has_active_for_session); passing session_id here used to
+                # never match, so active sessions got pruned anyway.
                 if self._has_active_processes_fn is not None:
                     try:
-                        if self._has_active_processes_fn(entry.session_id):
+                        if self._has_active_processes_fn(entry.session_key):
                             continue
-                    except Exception:
-                        pass
+                    except Exception as exc:
+                        logger.debug(
+                            "has_active_processes_fn raised during prune for %s: %s",
+                            entry.session_key, exc,
+                        )
                 if entry.updated_at < cutoff:
                     removed_keys.append(key)
             for key in removed_keys: