feat: Implement tool provenance tracking for tool execution logging

[vasilyl]

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Link to Issue or Description of Change

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

• Closes: N/A
• Related: feat: Add BigQuery Agent Analytics Plugin #738

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

Problem:
During complex multi-agent executions or nested tool invocations, it is difficult to trace the origin of a tool (e.g., whether it is a local function, an MCP tool, a sub-agent, or a transfer agent). This lack of provenance makes debugging and analytics on tool usage patterns highly challenging.

Solution:
Introduce a getToolOrigin(t tool.Tool) string helper inside bigquery_agent_analytics_plugin.go to inspect the tool's type and return its origin classification (LOCAL, SUB_AGENT, MCP, A2A, TRANSFER_AGENT, or UNKNOWN). Use this to populate a tool_origin attribute in telemetry logs for TOOL_START, TOOL_END, and TOOL_ERROR event types.

Testing Plan

Added a unit test TestToolProvenance to verify that getToolOrigin correctly identifies LOCAL function tools and SUB_AGENT agent tools.

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.
• I have manually tested my changes end-to-end.