Update ADK doc according to issue #1227 - 4 - Toolset Authentication

[adk-bot]

4. Document Toolset Authentication flow

Doc file: docs/tools-custom/authentication.md

Current state:

Describes "Journey 1" (configuring tools with auth) and "Journey 2" (handling auth inside tool function). Does not mention automatic proactive auth resolution for toolsets.

Proposed Change:

Add a new section or update "Journey 1" to explain "Toolset Authentication".
Explain that BaseLlmFlow now automatically checks BaseToolset.get_auth_config() before tools are listed or executed.
If credentials are missing, the framework pauses execution and issues an adk_request_credential event, just like in the tool-level auth flow.
This allows toolsets to define auth requirements centrally and have them resolved proactively.

Reasoning:
The new _resolve_toolset_auth method in BaseLlmFlow introduces a proactive authentication mechanism for toolsets, which is a significant change from the previous reactive (tool-level) or static configuration approaches.

Reference: src/google/adk/flows/llm_flows/base_llm_flow.py

[zyantw]

Closing with #2138