fix: lazy import strands-agents-evals to avoid ImportError when not installed (#367)

evaluation/__init__.py unconditionally imported StrandsEvalsAgentCoreEvaluator
and create_strands_evaluator, which transitively imports strands_evals. This
caused an ImportError for users who installed bedrock-agentcore without the
strands-agents-evals extra.

Use __getattr__ lazy loading so strands-dependent symbols are only imported
when actually accessed, with a clear error message if the extra is missing.

Closes #340

Author: jariy17

src/bedrock_agentcore/evaluation/__init__.py

@@ -1,10 +1,6 @@
 """AgentCore Evaluation: EvaluationClient and Strands integration."""
 
 from bedrock_agentcore.evaluation.client import EvaluationClient
-from bedrock_agentcore.evaluation.integrations.strands_agents_evals.evaluator import (
-    StrandsEvalsAgentCoreEvaluator,
-    create_strands_evaluator,
-)
 from bedrock_agentcore.evaluation.span_to_adot_serializer import (
     convert_strands_to_adot,
 )
@@ -19,3 +15,31 @@
     "create_strands_evaluator",
     "fetch_spans_from_cloudwatch",
 ]
+
+_STRANDS_EVALS_EXTRAS = {
+    "StrandsEvalsAgentCoreEvaluator",
+    "create_strands_evaluator",
+}
+
+
+def __getattr__(name: str):
+    """Lazy import for optional strands-agents-evals dependencies."""
+    if name in _STRANDS_EVALS_EXTRAS:
+        try:
+            from bedrock_agentcore.evaluation.integrations.strands_agents_evals.evaluator import (
+                StrandsEvalsAgentCoreEvaluator,
+                create_strands_evaluator,
+            )
+        except ImportError as e:
+            raise ImportError(
+                f"'{name}' requires the 'strands-agents-evals' extra. "
+                "Install it with: pip install 'bedrock-agentcore[strands-agents-evals]'"
+            ) from e
+
+        _lazy = {
+            "StrandsEvalsAgentCoreEvaluator": StrandsEvalsAgentCoreEvaluator,
+            "create_strands_evaluator": create_strands_evaluator,
+        }
+        return _lazy[name]
+
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")