refactor(bedrock): block_index resolves, callers own the current-index state

The helper wrote current_block_index for every event and the stop branch
immediately undid it; make it a pure resolver and assign in the start/delta
branches only. The test's replay tail re-covered ordering the sidecar
assertion already pins — dropped.

agent/bedrock_adapter.py

@@ -829,12 +829,10 @@ def stream_converse_with_callbacks(
     def block_index(payload: Dict[str, Any], *, new_block: bool = False) -> int:
         """Index of the block a contentBlock* event addresses. Without ``contentBlockIndex`` (test doubles,
         proxies) a start opens a fresh slot and a delta/stop continues the current one."""
-        nonlocal current_block_index
         idx = payload.get("contentBlockIndex")
-        if not isinstance(idx, int):
-            idx = len(stream_blocks) if new_block or current_block_index is None else current_block_index
-        current_block_index = idx
-        return idx
+        if isinstance(idx, int):
+            return idx
+        return len(stream_blocks) if new_block or current_block_index is None else current_block_index
 
     def flush_text() -> None:
         if current_text_buffer:
@@ -849,7 +847,7 @@ def stream_converse_with_callbacks(
             break
         if "contentBlockStart" in event:
             start_event = event["contentBlockStart"]
-            idx = block_index(start_event, new_block=True)
+            idx = current_block_index = block_index(start_event, new_block=True)
             start = start_event.get("start", {})
             if "toolUse" in start:
                 has_tool_use = True
@@ -860,7 +858,7 @@ def stream_converse_with_callbacks(
                     on_tool_start(current_tool["name"])
         elif "contentBlockDelta" in event:
             delta_event = event["contentBlockDelta"]
-            idx = block_index(delta_event)
+            idx = current_block_index = block_index(delta_event)
             delta = delta_event.get("delta", {})
             if "text" in delta:
                 text = delta["text"]

tests/agent/test_bedrock_adapter.py

@@ -508,7 +508,7 @@ class TestNormalizeConverseStreamEvents:
         the text into one block per delta and let the toolUse start (index 1) overwrite the second fragment
         and sort into the middle — a ``[text, toolUse, text...]`` sidecar Claude 5 on Bedrock rejects on
         replay as "does not support assistant message prefill"."""
-        from agent.bedrock_adapter import convert_messages_to_converse, normalize_converse_stream_events
+        from agent.bedrock_adapter import normalize_converse_stream_events
         result = normalize_converse_stream_events({"stream": list(self._LIVE_TEXT_THEN_TOOL_EVENTS)})
         msg = result.choices[0].message
         assert msg.content == "I'll echo banana now."
@@ -517,15 +517,6 @@ class TestNormalizeConverseStreamEvents:
             {"toolUse": {"toolUseId": "tooluse_1", "name": "echo", "input": {"s": "banana"}}},
         ]
         assert [tc.function.name for tc in msg.tool_calls] == ["echo"]
-        # The sidecar is authoritative on replay: the next turn must go out as text THEN toolUse, nothing after.
-        _, converse = convert_messages_to_converse([
-            {"role": "user", "content": "echo banana"},
-            {"role": "assistant", "content": msg.content, "bedrock_content_blocks": msg.bedrock_content_blocks,
-             "tool_calls": [{"id": "tooluse_1", "type": "function", "function": {"name": "echo", "arguments": '{"s": "banana"}'}}]},
-            {"role": "tool", "tool_call_id": "tooluse_1", "content": "banana"},
-        ])
-        assert [list(b)[0] for b in converse[1]["content"]] == ["text", "toolUse"]
-        assert converse[1]["content"][0]["text"] == "I'll echo banana now."
 
     def test_events_without_content_block_index_fall_back_to_arrival_order(self):
         """Proxies/test doubles may omit contentBlockIndex: deltas continue the current block, a start opens a