fix(bedrock): address review — route list-string items through _safe_text, update stale placeholder assertions

Addresses hermes-sweeper review on PR #66167:

1. _convert_content_to_converse() still emitted {"text": part} directly
   for plain-string items inside a content list (as opposed to
   {"type": "text"} dicts), bypassing _safe_text() entirely. A
   whitespace-only string item (e.g. ["   "]) could still reach Bedrock
   as a blank block. Now routed through _safe_text().

2. tests/agent/test_bedrock_adapter.py::TestEmptyTextBlockFix asserted
   the pre-fix behavior (whitespace -> literal space " "), contradicting
   the new _safe_text()/_EMPTY_TEXT_PLACEHOLDER behavior added in
   4618095a. Updated assertions to expect the non-whitespace placeholder,
   plus added a regression test for the list-string-item case above.

agent/bedrock_adapter.py

@@ -530,7 +530,7 @@ def _convert_content_to_converse(content) -> List[Dict]:
         blocks = []
         for part in content:
             if isinstance(part, str):
-                blocks.append({"text": part})
+                blocks.append({"text": _safe_text(part)})
                 continue
             if not isinstance(part, dict):
                 continue

tests/agent/test_bedrock_adapter.py

@@ -1332,28 +1332,46 @@ class TestIsAnthropicBedrockModel:
 
 
 class TestEmptyTextBlockFix:
-    """Test that empty text blocks are replaced with space placeholders."""
+    """Test that empty/whitespace-only text blocks are replaced with a
+    non-whitespace placeholder (not a literal space, which is itself
+    whitespace and gets rejected by the same Bedrock validation rule)."""
 
-    def test_none_content_gets_space(self):
-        from agent.bedrock_adapter import _convert_content_to_converse
+    def test_none_content_gets_placeholder(self):
+        from agent.bedrock_adapter import _convert_content_to_converse, _EMPTY_TEXT_PLACEHOLDER
         blocks = _convert_content_to_converse(None)
-        assert blocks[0]["text"] == " "
+        assert blocks[0]["text"] == _EMPTY_TEXT_PLACEHOLDER
+        assert blocks[0]["text"].strip()
 
-    def test_empty_string_gets_space(self):
-        from agent.bedrock_adapter import _convert_content_to_converse
+    def test_empty_string_gets_placeholder(self):
+        from agent.bedrock_adapter import _convert_content_to_converse, _EMPTY_TEXT_PLACEHOLDER
         blocks = _convert_content_to_converse("")
-        assert blocks[0]["text"] == " "
+        assert blocks[0]["text"] == _EMPTY_TEXT_PLACEHOLDER
+        assert blocks[0]["text"].strip()
 
-    def test_whitespace_only_gets_space(self):
-        from agent.bedrock_adapter import _convert_content_to_converse
+    def test_whitespace_only_gets_placeholder(self):
+        from agent.bedrock_adapter import _convert_content_to_converse, _EMPTY_TEXT_PLACEHOLDER
         blocks = _convert_content_to_converse("   ")
-        assert blocks[0]["text"] == " "
+        assert blocks[0]["text"] == _EMPTY_TEXT_PLACEHOLDER
+        assert blocks[0]["text"].strip()
 
     def test_real_text_preserved(self):
         from agent.bedrock_adapter import _convert_content_to_converse
         blocks = _convert_content_to_converse("Hello")
         assert blocks[0]["text"] == "Hello"
 
+    def test_whitespace_only_list_string_item_gets_placeholder(self):
+        """Regression: plain string items inside a content list (not
+        {"type": "text"} dicts) must also be routed through _safe_text()."""
+        from agent.bedrock_adapter import _convert_content_to_converse, _EMPTY_TEXT_PLACEHOLDER
+        blocks = _convert_content_to_converse(["   "])
+        assert blocks[0]["text"] == _EMPTY_TEXT_PLACEHOLDER
+        assert blocks[0]["text"].strip()
+
+    def test_real_list_string_item_preserved(self):
+        from agent.bedrock_adapter import _convert_content_to_converse
+        blocks = _convert_content_to_converse(["Hello"])
+        assert blocks[0]["text"] == "Hello"
+
 
 # ---------------------------------------------------------------------------
 # Stale-connection detection and per-region client invalidation