docs: add guide for MCP tool authentication and deployment

[lavinigam-gcp]

This PR addresses a significant documentation gap regarding the use of external, authenticated MCP servers with ADK.

The Problem

Currently, there is no documentation explaining how to pass authentication credentials (e.g., Bearer tokens) to a remote MCP server. While the underlying ADK framework supports this via the standard auth_scheme and auth_credential parameters on MCPToolset, this is not documented anywhere. This leaves users unable to integrate with secured, real-world MCP services.

Additionally, there is no guidance on how to deploy agents that use MCP tools to cloud environments like Cloud Run or Agent Engine, which presents challenges with process management and service-to-service authentication.

The Solution

This PR introduces a comprehensive solution:

1. New Guide: A new page, docs/tools/mcp-authentication.md, is created to serve as the single source of truth for this topic.
2. Code Example: A complete, runnable code example is added (agent_with_auth.py and mock_mcp_server.py), to demonstrate the pattern clearly.
3. Updated MCP Docs: The main mcp-tools.md page is updated to link to the new guide and modernize its connection parameter examples.
4. Updated Auth Docs: The core authentication.md page is updated to list MCPToolset as a supported tool for the ADK auth framework.
5. Updated Deployment Docs: Both the cloud-run.md and agent-engine.md guides are updated with new sections on best practices for deploying agents with MCP dependencies.

This change provides a clear, documented path for users to securely integrate ADK with external MCP tools and confidently deploy their agents to production environments.

Addresses #466

[ypenn21]

can you specify the dependencies that need to be installed?? I am having issues with imports:

from google.adk.auth import HttpAuth, HttpCredentials

from google.adk.auth.auth_schemes import HTTPBearer

my dependencies:

dependencies = [
"google-adk==1.6.1",
"python-dotenv==1.1.0",
"toolbox-core==0.1.0",
"gunicorn",
"whitenoise[brotli]",
"django>=5.0,<5.3",
"google-generativeai>=0.8.5",
"psycopg2-binary",
]

[polong-lin]

@lavinigam-gcp Could you PTAL at the merge conflict?

[enxilium]

can you specify the dependencies that need to be installed?? I am having issues with imports:

from google.adk.auth import HttpAuth, HttpCredentials

from google.adk.auth.auth_schemes import HTTPBearer

my dependencies:

dependencies = [ "google-adk==1.6.1", "python-dotenv==1.1.0", "toolbox-core==0.1.0", "gunicorn", "whitenoise[brotli]", "django>=5.0,<5.3", "google-generativeai>=0.8.5", "psycopg2-binary", ]

Same here, the imports don't seem to exist in the current version of the ADK.

[tpryan]

Repinnging @lavinigam-gcp to take a look at the merge conflict.