fix(langfuse): bound _TRACE_STATE growth from non-finalizing turns

Scoping the trace key by turn_id (the prior commit) fixed cross-turn
collisions but introduced a slow leak: _finish_trace only pops a key when a
turn ends cleanly (final response has content and no tool calls), so any
turn that is interrupted, ends on a tool call, or has empty final content
now leaves its uniquely-keyed entry in _TRACE_STATE forever. Previously the
constant per-session key was overwritten by the next turn, capping growth at
~1 entry per session.

Add an LRU cap (_MAX_TRACE_STATE) enforced by _evict_stale_locked, called
under _STATE_LOCK immediately before each insert. It evicts the
least-recently-updated entries (using the previously-dead last_updated_at
field) and ends their root span so nothing dangles. Regression test drives
50 non-finalizing turns against a cap of 8 and asserts the dict stays bounded
with the most-recent turns surviving.

tests/plugins/test_langfuse_plugin.py

@@ -373,6 +373,27 @@ class TestTurnTraceIsolation:
 
         assert k_pre_api == k_post_api == k_post_turn
 
+    def test_non_finalizing_turns_do_not_grow_state_unboundedly(self, monkeypatch):
+        """Per-turn keys mean a turn that never finalizes leaves a lingering
+        entry.  Without a cap that grows once per non-finalizing turn forever;
+        the LRU eviction must bound _TRACE_STATE at _MAX_TRACE_STATE.
+        """
+        mod = self._fresh_plugin()
+        started: list = []
+        monkeypatch.setattr(mod, "_get_langfuse", lambda: self._fake_client(started))
+        monkeypatch.setattr(mod, "_end_observation", lambda *a, **k: None)
+        monkeypatch.setattr(mod, "_MAX_TRACE_STATE", 8)
+        mod._TRACE_STATE.clear()
+
+        # Far more non-finalizing turns than the cap.
+        for n in range(50):
+            self._run_turn(mod, session="sess-leak", turn_n=n, finalize=False)
+
+        assert len(mod._TRACE_STATE) <= 8
+        # The survivors are the most-recently-updated turns (LRU eviction).
+        surviving = sorted(int(k.rsplit("turn", 1)[1]) for k in mod._TRACE_STATE)
+        assert surviving == list(range(42, 50))
+
     def test_trace_key_strings_unchanged_by_refactor(self):
         """Pin the exact key strings across all task/session/turn/api
         combinations so the _scope_prefix extraction can never silently change