feat: expose setup_complete event for ADK live

[lottielin]

This PR closes #2103 by exposing the BidiGenerateContentSetupComplete signal from Gemini Live API.

Currently, clients using run_live have no definitive way to know when the initial model setup is finished and the WebSocket is ready to receive user input.

This change:

• updates gemini_llm_connection.py to detect the setup_complete (see API)
• adds a setup_complete boolean field to the LlmResponse model
• modified _postprocess_live in base_llm_flow.py to allow setup_complete responses to pass through the event filter

Testing Plan

Unit Tests:

• added unit test test_receive_setup_complete: verify setup_complete messages are correctly adapted to LlmResponse
• added unit test test_live_streaming_setup_complete in test_streaming.py: verify run_live yields an event with setup_complete
• all unit tests pass locally

[gemini-code-assist]

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[ryanaiagent]

Hi @lottielin , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[ryanaiagent]

Hi @wyf7107 , can you please review this.