fix(ci): let the E2E proxy accept the mock testing params its suite sends

Gating the mock testing request params behind
general_settings.dangerously_allow_mock_testing_request_params (#35423) turned
every fallback, retry and timeout drill in tests/test_fallbacks.py into a 400:
the build_and_test job mounts proxy_server_config.yaml, which never opted in.

Opt that config in. It is the config the CI proxy runs with, and the suite it
serves exists to drive synthetic failures.

Add a unit test that ties the two together: it scans the top-level tests/test_*.py
files build_and_test globs for gated param names and fails if the config they run
against has not opted in, so the next change to either side is caught in a fast
lint-tier job rather than a Docker E2E.

proxy_server_config.yaml

@@ -227,6 +227,7 @@ general_settings:
   proxy_budget_rescheduler_max_time: 64
   proxy_batch_write_at: 1
   database_connection_pool_limit: 10
+  dangerously_allow_mock_testing_request_params: true
   # background_health_checks: true
   # use_shared_health_check: true  # needs a coordination Redis (below)
   # health_check_interval: 30

tests/test_litellm/proxy/test_route_llm_request.py

@@ -488,6 +488,45 @@ def test_gated_mock_params_cover_mock_router_testing_params():
     assert {"mock_testing_rate_limit_error", "mock_timeout", "mock_delay"} <= set(GATED_MOCK_PARAM_NAMES)
 
 
+def test_e2e_proxy_config_opts_in_to_the_mock_params_its_suite_sends():
+    """The ``build_and_test`` CI job runs the top-level ``tests/test_*.py`` suite
+    against a proxy mounted with ``proxy_server_config.yaml``. Several of those
+    tests drive fallback, retry and timeout paths by asking the proxy to
+    fabricate a failure, which the gate rejects with a 400 unless the config
+    opts in. Without the opt-in the positive cases fail outright and the
+    negative ones pass for the wrong reason, so the suite and the config it
+    runs against have to move together."""
+    from pathlib import Path
+
+    import yaml
+
+    from litellm.proxy.route_llm_request import (
+        GATED_MOCK_PARAM_NAMES,
+        MOCK_TESTING_CONFIG_KEY,
+    )
+
+    repo_root = Path(__file__).parents[3]
+    suite_sources = tuple(
+        (path, path.read_text(encoding="utf-8")) for path in sorted(repo_root.glob("tests/test_*.py"))
+    )
+    senders = frozenset(
+        f"{path.name}:{param}"
+        for path, source in suite_sources
+        for param in GATED_MOCK_PARAM_NAMES
+        if f"{param}=" in source or f'"{param}"' in source
+    )
+    assert senders, "expected the E2E suite to still exercise the gated mock testing params"
+
+    config = yaml.safe_load((repo_root / "proxy_server_config.yaml").read_text(encoding="utf-8"))
+    general_settings = config.get("general_settings") or {}
+
+    assert general_settings.get(MOCK_TESTING_CONFIG_KEY) is True, (
+        f"proxy_server_config.yaml must set general_settings.{MOCK_TESTING_CONFIG_KEY}: true — "
+        f"the E2E suite sends gated mock testing params ({', '.join(sorted(senders))}) "
+        "and the proxy rejects them with a 400 otherwise"
+    )
+
+
 @pytest.mark.parametrize(
     "mock_param",
     [