fix(tests): fix 11 real test failures + major cascade poisoner (#4570)

Three root causes addressed:

1. AIAgent no longer defaults base_url to OpenRouter (9 tests)
   Tests that assert OpenRouter-specific behavior (prompt caching,
   reasoning extra_body, provider preferences) need explicit base_url
   and model set on the agent. Updated test_run_agent.py and
   test_provider_parity.py.

2. Credential pool auto-seeding from host env (2 tests)
   test_auxiliary_client.py tests for Anthropic OAuth and custom
   endpoint fallback were not mocking _select_pool_entry, so the
   host's credential pool interfered. Added pool + codex mocks.

3. sys.modules corruption cascade (major - ~250 tests)
   test_managed_modal_environment.py replaced sys.modules entries
   (tools, hermes_cli, agent packages) with SimpleNamespace stubs
   but had NO cleanup fixture. Every subsequent test in the process
   saw corrupted imports: 'cannot import get_config_path from
   <unknown module name>' and 'module tools has no attribute
   environments'. Added _restore_tool_and_agent_modules autouse
   fixture matching the pattern in test_managed_browserbase_and_modal.py.

   This was also the root cause of CI failures (104 failed on main).

tests/test_provider_parity.py

@@ -73,6 +73,7 @@ class TestBuildApiKwargsOpenRouter:
 
     def test_includes_reasoning_in_extra_body(self, monkeypatch):
         agent = _make_agent(monkeypatch, "openrouter")
+        agent.model = "anthropic/claude-sonnet-4-20250514"
         messages = [{"role": "user", "content": "hi"}]
         kwargs = agent._build_api_kwargs(messages)
         extra = kwargs.get("extra_body", {})
@@ -798,6 +799,7 @@ class TestReasoningEffortDefaults:
 
     def test_openrouter_default_medium(self, monkeypatch):
         agent = _make_agent(monkeypatch, "openrouter")
+        agent.model = "anthropic/claude-sonnet-4-20250514"
         kwargs = agent._build_api_kwargs([{"role": "user", "content": "hi"}])
         reasoning = kwargs["extra_body"]["reasoning"]
         assert reasoning["effort"] == "medium"
@@ -825,6 +827,7 @@ class TestReasoningEffortDefaults:
 
     def test_openrouter_reasoning_config_override(self, monkeypatch):
         agent = _make_agent(monkeypatch, "openrouter")
+        agent.model = "anthropic/claude-sonnet-4-20250514"
         agent.reasoning_config = {"enabled": True, "effort": "medium"}
         kwargs = agent._build_api_kwargs([{"role": "user", "content": "hi"}])
         assert kwargs["extra_body"]["reasoning"]["effort"] == "medium"