Filter duplicate final payloads without extra hashing

[contextablemark]

Summary

• track streamed assistant output per run and reuse the existing usage_metadata guard to decide when to synthesize events
• emit non-streamed final responses only when their content differs from what was already streamed, while keeping callback completions intact
• update the comprehensive translator tests to cover duplicate suppression and replaying of genuinely new final payloads

Testing

• PYTHONPATH=sdks/python:integrations/adk-middleware/python/src pytest integrations/adk-middleware/python/tests/test_event_translator_comprehensive.py -q (fails: ModuleNotFoundError: No module named 'google')

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https://chatgpt.com/codex/tasks/task_e_68f253c87d888321af94faa7388e151e