feat: Pass rich structured event types to OnEventCallback (StateDelta, HITL, A2A)

[vasilyl]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: N/A

2. Or, if no issue exists, describe the change:

Problem:
Standard event logging only records basic string/json summaries of agent execution. Rich interactions like human-in-the-loop (HITL) approvals (credentials, confirmations, input), state delta modifications, and agent-to-agent interactions are not captured with structure, making analytical queries difficult.

Solution:
Refactor OnEventCallback in the analytics plugin to detect and parse structured event types: STATE_DELTA updates, HITL request events, and Agent-to-Agent (A2A) interaction statistics, serializing them into clear attributes and content blocks.

Testing Plan

Unit tests verify structured schemas are built properly inside bigquery_agent_analytics_plugin_test.go when invoking these callbacks.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
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