test(e2e): enforce the cross-region invariant on the Bedrock allowlist

The allowlist rejects an unlisted model before the region resolver runs, so the
two negative cases that used to cover the resolver were passing for the wrong
reason and two mutations of it survived. Answering an env-referenced region with
the default mount is only sound because every allowlisted model is a `us.`
profile that fans out across the US regions, so assert that on the list itself
and drop the per-call branch it made unreachable.

tests/code_coverage_tests/test_provider_cache.py

@@ -33,7 +33,13 @@ from provider_cache import (
     successful_response,
 )
 from provider_cache_redis import PUBLISH, RedisCommands, RedisResponseStore, configured_cache, redis_store
-from provider_cache_routing import LIVE_PROVIDER_REQUIRED, route_cache_model
+from provider_cache_routing import (
+    BEDROCK_CROSS_REGION_PREFIX,
+    BEDROCK_EDGE_MODELS,
+    LIVE_PROVIDER_REQUIRED,
+    bedrock_region,
+    route_cache_model,
+)
 from fixture_mode import SESSION_TEST_KEY
 from provider_edge import EDGE_MOUNTS, configured_cache_backend, resolve_mount, start_provider_edge
 from provider_edge_bedrock import bedrock_signer
@@ -856,6 +862,31 @@ def test_bedrock_deployments_the_edge_must_not_touch_keep_their_direct_route(par
     assert routed is params
 
 
+@pytest.mark.parametrize("declared,expected", [
+    (None, "us-east-1"),
+    ("us-west-2", "us-west-2"),
+    ("eu-west-1", "eu-west-1"),
+    ("os.environ/AWS_REGION", "us-east-1"),
+    ("os.environ/ANY_OTHER_NAME", "us-east-1"),
+])
+def test_a_region_only_the_proxy_can_resolve_falls_back_to_the_default_mount(
+    declared: str | None, expected: str,
+) -> None:
+    """A declared literal region is the one the deployment meant. A region the
+    proxy resolves from its own environment is one the run pod cannot see, and
+    the default mount answers it."""
+    assert bedrock_region(declared) == expected
+
+
+def test_every_model_on_the_edge_allowlist_is_a_cross_region_profile() -> None:
+    """Answering an env-referenced region with the default mount is only correct
+    for a profile that fans out across the US regions and is reachable from any
+    of them. A single-region model on this list would be sent to a region it may
+    not exist in, so the list is where that is caught."""
+    assert BEDROCK_EDGE_MODELS
+    assert all(model.startswith(BEDROCK_CROSS_REGION_PREFIX) for model in BEDROCK_EDGE_MODELS)
+
+
 @pytest.mark.parametrize("mode", ["batch", "realtime", "image_generation"])
 def test_a_bedrock_deployment_with_a_mode_keeps_its_direct_route(mode: ModelMode) -> None:
     params: Final = LiteLLMParamsBody(model="bedrock/us.anthropic.claude-haiku-4-5-20251001-v1:0")

tests/e2e/provider_cache_routing.py

@@ -21,22 +21,18 @@ BEDROCK_EDGE_MODELS: Final = frozenset(
 ENV_REFERENCE_PREFIX: Final = "os.environ/"
 
 
-def bedrock_region(declared: str | None, model: str) -> str | None:
-    """The region whose edge mount a deployment belongs to, or None when the
-    harness cannot know it.
+def bedrock_region(declared: str | None) -> str:
+    """The region whose edge mount a deployment belongs to.
 
-    Most Bedrock deployments declare `os.environ/AWS_REGION`, which the proxy
-    resolves from its own environment. The run pod does not share that
-    environment, so the harness genuinely does not know the region. A `us.`
-    inference profile fans out across the US regions and is reachable from any
-    of them, so the default entry point is correct for those whatever the proxy
-    resolved; anything else keeps its direct path rather than being sent to a
-    region the model may not exist in."""
-    if declared is None:
+    Most Bedrock deployments declare `os.environ/AWS_REGION`, which only the
+    proxy can resolve from its own environment; the run pod does not share it.
+    Answering those with the default mount is correct because every model on the
+    edge allowlist is a `us.` inference profile, which fans out across the US
+    regions and is reachable from any of them. That invariant is enforced on the
+    allowlist itself rather than re-checked per call."""
+    if declared is None or declared.startswith(ENV_REFERENCE_PREFIX):
         return DEFAULT_BEDROCK_REGION
-    if not declared.startswith(ENV_REFERENCE_PREFIX):
-        return declared
-    return DEFAULT_BEDROCK_REGION if model.startswith(BEDROCK_CROSS_REGION_PREFIX) else None
+    return declared
 
 
 def bedrock_mount(params: LiteLLMParamsBody) -> str | None:
@@ -51,8 +47,7 @@ def bedrock_mount(params: LiteLLMParamsBody) -> str | None:
     model: Final = route.partition("/")[2] or route
     if model not in BEDROCK_EDGE_MODELS:
         return None
-    region: Final = bedrock_region(params.aws_region_name, model)
-    return None if region is None else f"bedrock/{region}"
+    return f"bedrock/{bedrock_region(params.aws_region_name)}"
 
 
 def route_bedrock(