fix(anthropic): fix duplicate ToolCallStartEvent in streaming tool calls (#3051)

When processing `input_json_delta` events, the Anthropic client was
passing the tool name from the previous `tool_use` event. This caused
ag-ui's `_handle_function_call_content` to emit a `ToolCallStartEvent`
for every streaming chunk (since it triggers on `if content.name:`).

This fix changes the behavior to pass an empty string for `name` in
`input_json_delta` events, matching OpenAI's behavior where streaming
argument chunks have `name=""`. The initial `tool_use` event still
provides the tool name, so only one `ToolCallStartEvent` is emitted.

Co-authored-by: Evan Mattson <35585003+moonbox3@users.noreply.github.com>

python/packages/ag-ui/tests/test_events_comprehensive.py

@@ -152,6 +152,42 @@ async def test_tool_call_streaming_args():
     assert events1[0].tool_call_id == events2[0].tool_call_id == events3[0].tool_call_id
 
 
+async def test_streaming_tool_call_no_duplicate_start_events():
+    """Test that streaming tool calls emit exactly one ToolCallStartEvent.
+
+    This is a regression test for the Anthropic streaming fix where input_json_delta
+    events were incorrectly passing the tool name, causing duplicate ToolCallStartEvents.
+
+    The correct behavior is:
+    - Initial FunctionCallContent with name -> emits ToolCallStartEvent
+    - Subsequent FunctionCallContent with name="" -> emits only ToolCallArgsEvent
+
+    See: https://github.com/microsoft/agent-framework/pull/3051
+    """
+    from agent_framework_ag_ui._events import AgentFrameworkEventBridge
+
+    bridge = AgentFrameworkEventBridge(run_id="test_run", thread_id="test_thread")
+
+    # Simulate streaming tool call: first chunk has name, subsequent chunks have name=""
+    update1 = AgentRunResponseUpdate(contents=[FunctionCallContent(name="get_weather", call_id="call_789")])
+    update2 = AgentRunResponseUpdate(contents=[FunctionCallContent(name="", call_id="call_789", arguments='{"loc":')])
+    update3 = AgentRunResponseUpdate(contents=[FunctionCallContent(name="", call_id="call_789", arguments='"SF"}')])
+
+    events1 = await bridge.from_agent_run_update(update1)
+    events2 = await bridge.from_agent_run_update(update2)
+    events3 = await bridge.from_agent_run_update(update3)
+
+    # Count all ToolCallStartEvents - should be exactly 1
+    all_events = events1 + events2 + events3
+    tool_call_start_count = sum(1 for e in all_events if e.type == "TOOL_CALL_START")
+    assert tool_call_start_count == 1, f"Expected 1 ToolCallStartEvent, got {tool_call_start_count}"
+
+    # Verify event types
+    assert events1[0].type == "TOOL_CALL_START"
+    assert events2[0].type == "TOOL_CALL_ARGS"
+    assert events3[0].type == "TOOL_CALL_ARGS"
+
+
 async def test_tool_result_with_dict():
     """Test FunctionResultContent with dict result."""
     from agent_framework_ag_ui._events import AgentFrameworkEventBridge