Update ADK doc according to issue #1587 - 5

[adk-bot]

5. Vertex AI Model Names

Doc file: docs/agents/models/vertex.md

Current state:

The documentation states how to use Vertex AI by setting environment variables and providing the base model name (like gemini-2.5-flash or claude-3-5-sonnet@20240620). It does not mention that fully qualified Vertex resource names are directly supported as model names.

Proposed Change:

Add a section or note explaining that developers can now pass fully qualified Vertex AI model or endpoint resource names (e.g., projects/YOUR_PROJECT/locations/YOUR_REGION/publishers/google/models/gemini-2.5-flash or projects/.../endpoints/YOUR_ENDPOINT_ID) directly into the model parameter when creating an Agent, a Gemini instance, or an Anthropic Claude instance. Explain that ADK will automatically parse the Project ID, Location, and base model name from this string, and properly configure the SDK client for Vertex AI mode without requiring external environment variables.

Reasoning:
This is a major quality-of-life and usability improvement that allows developers to seamlessly drop in Vertex AI endpoint strings directly into their code. This is particularly useful when working with Model Garden endpoints or switching between different projects.

Reference: src/google/adk/models/google_llm.py

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