Python: Fix doubled tool_call arguments in MESSAGES_SNAPSHOT when streaming (#4200)

* fix: prevent doubled tool_call arguments in MESSAGES_SNAPSHOT

When streaming with client-side tools, some providers send a full-
arguments replay after the streaming deltas complete. The `_emit_tool_call`
function unconditionally appends every arguments delta to the internal
`flow.tool_calls_by_id` tracking dictionary via `+=`. When the replay
contains the exact same complete arguments string that was already
accumulated from prior deltas, the arguments get doubled (e.g.,
`{"todoText":"buy groceries"}{"todoText":"buy groceries"}`).

This causes `MESSAGES_SNAPSHOT` events to contain invalid doubled JSON in
`tool_calls[].function.arguments`, breaking any client or middleware that
relies on snapshots for state reconstruction.

The fix adds a guard (mirroring the existing duplicate guard in
`_emit_text`) that detects when the incoming delta exactly equals the
already-accumulated arguments string, indicating a full-arguments replay
rather than an incremental delta. In this case the append is skipped,
preventing the doubling.

The `ToolCallArgsEvent` deltas are still emitted correctly for real-time
streaming — only the internal snapshot accumulator is guarded.

Fixes #4194

* fix: move duplicate check before event emission + add test

Address Copilot review feedback:
1. Move duplicate full-arguments replay detection BEFORE emitting
   ToolCallArgsEvent, for consistency with _emit_text() which returns
   early without emitting any events on replay detection.
2. Add test_emit_tool_call_skips_duplicate_full_arguments_replay() to
   verify the duplicate detection behavior for tool call arguments,
   matching the existing test pattern for text content.

python/packages/ag-ui/tests/ag_ui/test_run.py

@@ -6,6 +6,7 @@ import pytest
 from ag_ui.core import (
     TextMessageEndEvent,
     TextMessageStartEvent,
+    ToolCallArgsEvent,
 )
 from agent_framework import AgentResponseUpdate, Content, Message, ResponseStream
 from agent_framework.exceptions import AgentInvalidResponseException
@@ -416,6 +417,42 @@ def test_emit_tool_call_generates_id():
     assert flow.tool_call_id is not None  # ID should be generated
 
 
+def test_emit_tool_call_skips_duplicate_full_arguments_replay():
+    """Test _emit_tool_call skips replayed full-arguments on an existing tool call.
+
+    This is a regression test for issue #4194 where some streaming providers
+    send the full arguments string again after streaming deltas, causing the
+    arguments to be doubled in MESSAGES_SNAPSHOT events.
+
+    Mirrors test_emit_text_skips_duplicate_full_message_delta for consistency.
+    """
+    flow = FlowState()
+    full_args = '{"city": "Seattle"}'
+
+    # Step 1: Initial tool call with name + arguments (normal start)
+    content_start = Content.from_function_call(
+        call_id="call_dup",
+        name="get_weather",
+        arguments=full_args,
+    )
+    events_start = _emit_tool_call(content_start, flow)
+
+    # Should emit ToolCallStartEvent + ToolCallArgsEvent
+    assert any(isinstance(e, ToolCallArgsEvent) for e in events_start)
+    assert flow.tool_calls_by_id["call_dup"]["function"]["arguments"] == full_args
+
+    # Step 2: Provider replays the full arguments (duplicate)
+    content_replay = Content(type="function_call", call_id="call_dup", arguments=full_args)
+    events_replay = _emit_tool_call(content_replay, flow)
+
+    # Should NOT emit any ToolCallArgsEvent (early return on replay)
+    args_events = [e for e in events_replay if isinstance(e, ToolCallArgsEvent)]
+    assert args_events == [], "Duplicate full-arguments replay should not emit ToolCallArgsEvent"
+
+    # Accumulated arguments should remain unchanged
+    assert flow.tool_calls_by_id["call_dup"]["function"]["arguments"] == full_args
+
+
 def test_emit_tool_result_closes_open_message():
     """Test _emit_tool_result emits TextMessageEndEvent for open text message.