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/agent_framework_ag_ui/_run_common.py

@@ -195,6 +195,23 @@ def _emit_tool_call(
         delta = (
             content.arguments if isinstance(content.arguments, str) else json.dumps(make_json_safe(content.arguments))
         )
+
+        if tool_call_id in flow.tool_calls_by_id:
+            accumulated = flow.tool_calls_by_id[tool_call_id]["function"]["arguments"]
+            # Guard against full-argument replay: if the accumulated arguments
+            # already equal the incoming delta, this is a non-delta replay of
+            # the complete arguments string (some providers send the full
+            # arguments again after streaming deltas). Skip the event emission
+            # and accumulation to prevent doubling in MESSAGES_SNAPSHOT.
+            # This mirrors the early-return behaviour of _emit_text().
+            # (Fixes #4194)
+            if accumulated and delta == accumulated:
+                logger.debug(
+                    "Skipping duplicate full-arguments replay for tool_call_id=%s",
+                    tool_call_id,
+                )
+                return events
+
         events.append(ToolCallArgsEvent(tool_call_id=tool_call_id, delta=delta))
 
         if tool_call_id in flow.tool_calls_by_id: