fix(agent): accept empty content with stop_reason=end_turn as valid anthropic response

Anthropic's API can legitimately return content=[] with stop_reason="end_turn"
when the model has nothing more to add after a turn that already delivered the
user-facing text alongside a trivial tool call (e.g. memory write). The transport
validator was treating that as an invalid response, triggering 3 retries that
each returned the same valid-but-empty response, then failing the run with
"Invalid API response after 3 retries."

The downstream normalizer already handles empty content correctly (empty loop
over response.content, content=None, finish_reason="stop"), so the only fix
needed is at the validator boundary.

Tests:
- Empty content + stop_reason="end_turn" → valid (the fix)
- Empty content + stop_reason="tool_use" → still invalid (regression guard)
- Empty content without stop_reason → still invalid (existing behavior preserved)

tests/agent/transports/test_transport.py

@@ -114,6 +114,14 @@ class TestAnthropicTransport:
         r = SimpleNamespace(content=[])
         assert transport.validate_response(r) is False
 
+    def test_validate_response_empty_content_with_end_turn_is_valid(self, transport):
+        r = SimpleNamespace(content=[], stop_reason="end_turn")
+        assert transport.validate_response(r) is True
+
+    def test_validate_response_empty_content_with_tool_use_is_invalid(self, transport):
+        r = SimpleNamespace(content=[], stop_reason="tool_use")
+        assert transport.validate_response(r) is False
+
     def test_validate_response_valid(self, transport):
         r = SimpleNamespace(content=[SimpleNamespace(type="text", text="hello")])
         assert transport.validate_response(r) is True