test(gateway): cover fallback_providers reload for live sessions

Pin reload + cached-agent apply helpers for #60955 so a mid-uptime
fallback chain change reaches messaging sessions without a restart.

(cherry picked from commit fafb341035)

tests/gateway/test_fallback_chain_reload.py

@@ -0,0 +1,165 @@
+"""Regression tests for #60955: gateway must not freeze fallback_providers.
+
+Cron reloads ``fallback_providers`` from disk on every job. The gateway used to
+freeze ``self._fallback_model`` at process start, so a chain configured (or
+edited) after ``hermes gateway`` was already running never reached messaging
+sessions — even though cron in the same process fell back correctly.
+
+These tests pin the reload + cached-agent apply helpers without driving the
+full Feishu session path.
+"""
+
+from __future__ import annotations
+
+import time
+from types import SimpleNamespace
+
+
+def test_refresh_fallback_model_rereads_config(tmp_path, monkeypatch):
+    from gateway.run import GatewayRunner
+
+    monkeypatch.setattr("gateway.run._hermes_home", tmp_path)
+    cfg = tmp_path / "config.yaml"
+    cfg.write_text(
+        "fallback_providers:\n"
+        "  - provider: deepseek\n"
+        "    model: deepseek-v4-flash\n"
+    )
+
+    runner = SimpleNamespace(
+        _fallback_model=None,
+    )
+    runner._load_fallback_model = GatewayRunner._load_fallback_model
+    bound = GatewayRunner._refresh_fallback_model.__get__(runner)
+    chain = bound()
+
+    assert chain == [{"provider": "deepseek", "model": "deepseek-v4-flash"}]
+    assert runner._fallback_model == chain
+
+    cfg.write_text(
+        "fallback_providers:\n"
+        "  - provider: openrouter\n"
+        "    model: anthropic/claude-sonnet-4.6\n"
+    )
+    updated = bound()
+    assert updated == [
+        {"provider": "openrouter", "model": "anthropic/claude-sonnet-4.6"}
+    ]
+    assert runner._fallback_model == updated
+
+
+def test_refresh_fallback_model_clears_when_config_removed(tmp_path, monkeypatch):
+    from gateway.run import GatewayRunner
+
+    monkeypatch.setattr("gateway.run._hermes_home", tmp_path)
+    cfg = tmp_path / "config.yaml"
+    cfg.write_text(
+        "fallback_providers:\n"
+        "  - provider: deepseek\n"
+        "    model: deepseek-v4-flash\n"
+    )
+
+    runner = SimpleNamespace(
+        _fallback_model=[{"provider": "stale", "model": "x"}],
+    )
+    runner._load_fallback_model = GatewayRunner._load_fallback_model
+    bound = GatewayRunner._refresh_fallback_model.__get__(runner)
+    assert bound() is not None
+
+    cfg.write_text("model:\n  provider: nvidia\n")
+    assert bound() is None
+    assert runner._fallback_model is None
+
+
+def test_apply_fallback_chain_updates_primary_agent():
+    from gateway.run import GatewayRunner
+
+    agent = SimpleNamespace(
+        _fallback_chain=[],
+        _fallback_model=None,
+        _fallback_index=0,
+        _fallback_activated=False,
+        _rate_limited_until=0,
+    )
+    chain = [{"provider": "deepseek", "model": "deepseek-v4-flash"}]
+    GatewayRunner._apply_fallback_chain_to_agent(agent, chain)
+
+    assert agent._fallback_chain == chain
+    assert agent._fallback_model == chain[0]
+    assert agent._fallback_index == 0
+
+
+def test_apply_fallback_chain_skips_while_cooldown_holds_fallback():
+    """Do not clobber a live fallback activation during its cooldown window."""
+    from gateway.run import GatewayRunner
+
+    live = [{"provider": "deepseek", "model": "deepseek-v4-flash"}]
+    agent = SimpleNamespace(
+        _fallback_chain=live,
+        _fallback_model=live[0],
+        _fallback_index=1,
+        _fallback_activated=True,
+        _rate_limited_until=time.monotonic() + 30,
+    )
+    GatewayRunner._apply_fallback_chain_to_agent(
+        agent,
+        [{"provider": "openrouter", "model": "anthropic/claude-sonnet-4.6"}],
+    )
+
+    assert agent._fallback_chain == live
+    assert agent._fallback_index == 1
+    assert agent._fallback_activated is True
+
+
+def test_apply_fallback_chain_updates_after_cooldown_expires():
+    from gateway.run import GatewayRunner
+
+    agent = SimpleNamespace(
+        _fallback_chain=[{"provider": "deepseek", "model": "old"}],
+        _fallback_model={"provider": "deepseek", "model": "old"},
+        _fallback_index=1,
+        _fallback_activated=True,
+        _rate_limited_until=time.monotonic() - 1,
+    )
+    new_chain = [{"provider": "openrouter", "model": "anthropic/claude-sonnet-4.6"}]
+    GatewayRunner._apply_fallback_chain_to_agent(agent, new_chain)
+
+    assert agent._fallback_chain == new_chain
+    assert agent._fallback_model == new_chain[0]
+    # Activated agents keep their index; restore_primary_runtime owns reset.
+    assert agent._fallback_index == 1
+
+
+def test_background_and_main_agent_paths_call_refresh():
+    """Both AIAgent construction sites must pass a refreshed chain, not the
+    startup snapshot. Source-level invariant — mirrors
+    tests/agent/test_gemini_fast_fallback.py's call-site pin.
+    """
+    from pathlib import Path
+
+    source = Path("gateway/run.py").read_text(encoding="utf-8")
+    assert "fallback_model=self._refresh_fallback_model()" in source
+    assert source.count("fallback_model=self._refresh_fallback_model()") >= 2
+    # The stale startup-snapshot form must not remain at create sites.
+    assert "fallback_model=self._fallback_model," not in source
+
+
+def test_load_fallback_model_static_unchanged_contract(tmp_path, monkeypatch):
+    """_load_fallback_model remains a pure static reader used by refresh."""
+    from gateway.run import GatewayRunner
+
+    monkeypatch.setattr("gateway.run._hermes_home", tmp_path)
+    (tmp_path / "config.yaml").write_text(
+        "fallback_providers:\n"
+        "  - provider: deepseek\n"
+        "    model: deepseek-v4-flash\n"
+        "fallback_model:\n"
+        "  provider: nous\n"
+        "  model: Hermes-4\n"
+    )
+
+    chain = GatewayRunner._load_fallback_model()
+    assert chain == [
+        {"provider": "deepseek", "model": "deepseek-v4-flash"},
+        {"provider": "nous", "model": "Hermes-4"},
+    ]