fix: reconcile probe cache with stale-entry invalidation + stale test fixtures

- The #44861 stale-cache guard invalidated any cached value that differed
  from the static table, which would have discarded legitimate
  probe-derived windows larger than the table. Treat the table as a
  FLOOR: only drop under-reporting cache entries.
- Update probe test fixtures that predated the 4.6+ 1M table flip
  (opus-4-6 fallback expectations 200K -> 1M).

agent/model_metadata.py

@@ -2250,12 +2250,16 @@ def get_model_context_length(
                 )
                 # Fall through; step 5b reconciles and overwrites if portal responds.
             # Invalidate stale Bedrock entries seeded before the Claude 4.6+
-            # long-context table was corrected to 1M.
+            # long-context table was corrected to 1M. The static table is a
+            # FLOOR, not an override: probe-derived cache entries (step 1b)
+            # may legitimately exceed the table (real window read from
+            # Bedrock's length-validation error), so only under-reporting
+            # entries are dropped — never a cached value above the table.
             elif is_bedrock_context:
                 try:
                     from agent.bedrock_adapter import get_bedrock_context_length
                     bedrock_ctx = get_bedrock_context_length(model)
-                    if cached != bedrock_ctx:
+                    if cached < bedrock_ctx:
                         logger.info(
                             "Dropping stale Bedrock cache entry %s@%s -> %s; "
                             "using static Bedrock table value %s",

tests/agent/test_bedrock_adapter.py

@@ -1244,7 +1244,7 @@ class TestBedrockContextLength:
         # still invoke get_bedrock_context_length(model_id) with one arg.
         from agent.bedrock_adapter import get_bedrock_context_length
         with patch("agent.bedrock_adapter.probe_bedrock_context_length") as mock_probe:
-            assert get_bedrock_context_length("anthropic.claude-opus-4-6") == 200_000
+            assert get_bedrock_context_length("anthropic.claude-opus-4-6") == 1_000_000
             mock_probe.assert_not_called()
 
 
@@ -1299,9 +1299,9 @@ class TestBedrockContextProbe:
         err = "AccessDeniedException: nope"
         with patch("agent.bedrock_adapter._get_bedrock_runtime_client",
                    return_value=self._client_raising(err)):
-            # opus-4-6 is in the table at 200K; probe fails → table wins.
+            # opus-4-6 is in the table at 1M; probe fails → table wins.
             assert get_bedrock_context_length(
-                "anthropic.claude-opus-4-6", region="eu-central-1") == 200_000
+                "anthropic.claude-opus-4-6", region="eu-central-1") == 1_000_000
 
 
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