fix: Replace Event ID generation with UUID4 to prevent SQLite integrity constraint failures

Fixes #719 where duplicate Event IDs caused database constraint violations
during streaming responses. Replaces 8-character random string generation
with UUID4 to ensure globally unique identifiers.

Changes:
- Replace Event.new_id() random string approach with uuid.uuid4()
- Remove unused random and string imports
- Add uuid import

Eliminates SQLite UNIQUE constraint failures in streaming scenarios.

src/google/adk/events/event.py

@@ -14,9 +14,8 @@
 from __future__ import annotations
 
 from datetime import datetime
-import random
-import string
 from typing import Optional
+import uuid
 
 from google.genai import types
 from pydantic import alias_generators
@@ -132,5 +131,4 @@ class Event(LlmResponse):
 
   @staticmethod
   def new_id():
-    characters = string.ascii_letters + string.digits
-    return ''.join(random.choice(characters) for _ in range(8))
+    return str(uuid.uuid4())