Update ADK doc according to issue #1743 - 2

[adk-bot]

2. Document AGENT_RESPONSE in BigQuery Analytics Event Summary Table

Doc file: docs/integrations/bigquery-agent-analytics.md

Current state:

| AGENT_STARTING | Agent execution begins | instruction summary | v_agent_starting |
| AGENT_COMPLETED | Agent execution ends | latency | v_agent_completed |
| LLM_REQUEST | A model request is sent | model, prompt, config, tools | v_llm_request |

Proposed Change:

| AGENT_STARTING | Agent execution begins | instruction summary | v_agent_starting |
| AGENT_COMPLETED | Agent execution ends | latency | v_agent_completed |
| AGENT_RESPONSE | Final response text emitted by the agent | response text, source event ID | v_agent_response |
| LLM_REQUEST | A model request is sent | model, prompt, config, tools | v_llm_request |

Reasoning:
The BigQuery Agent Analytics Plugin now detects and logs final responses emitted by agents as a distinct AGENT_RESPONSE event type, which captures the actual visible response text. This needs to be documented in the event summary table.

Reference: src/google/adk/plugins/bigquery_agent_analytics_plugin.py

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[jcwriter74]

• Related Document v_agent_response View in BigQuery Analytics #1743 - 3 #1747
• Added changes in here
• Also on https://adk.dev/integrations/bigquery-agent-analytics/#:~:text=JSON%20payload%20data.-,Captured%20events%20summary,-%C2%B6
• Not on google cloud docs