fix(gateway): cap cached session sources with LRU eviction

Follow-up on top of Zyproth's session-source cache: swap the unbounded
dict for an OrderedDict with a 512-entry LRU cap so long-running
gateways can't accumulate stale entries for dead sessions forever.

- self._session_sources is now an OrderedDict
- _cache_session_source() move_to_end + popitem(last=False) above cap
- _get_cached_session_source() move_to_end on hit (LRU read bump)
- restart_test_helpers.py wires OrderedDict + _session_sources_max

tests/gateway/restart_test_helpers.py

@@ -1,4 +1,5 @@
 import asyncio
+from collections import OrderedDict
 from unittest.mock import AsyncMock, MagicMock
 
 from gateway.config import GatewayConfig, Platform, PlatformConfig
@@ -74,7 +75,8 @@ def make_restart_runner(
     runner._update_prompt_pending = {}
     runner._voice_mode = {}
     runner._session_model_overrides = {}
-    runner._session_sources = {}
+    runner._session_sources = OrderedDict()
+    runner._session_sources_max = 512
     runner._shutdown_all_gateway_honcho = lambda: None
     runner._update_runtime_status = MagicMock()
     runner._queue_or_replace_pending_event = GatewayRunner._queue_or_replace_pending_event.__get__(