Link to Issue or Description of Change

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

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

Problem:
The current AgentRegistry client uses httpx.Client for API requests, which does not inherently support the mTLS (mutual TLS) requirements and automatic endpoint selection needed for secure Google API interactions.

Solution:
Integrated mTLS support by transitioning the AgentRegistry client to use google.auth.transport.requests.AuthorizedSession. This change allows the client to:

• Automatically handle mTLS configuration based on environment variables like GOOGLE_API_USE_CLIENT_CERTIFICATE and GOOGLE_API_USE_MTLS_ENDPOINT.
• Dynamically select the correct base URL between the standard (agentregistry.googleapis.com) and mTLS (agentregistry.mtls.googleapis.com) endpoints.
• Leverage standard Google Auth libraries for secure credential management and session handling.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Please include a summary of passed pytest results.

Manual End-to-End (E2E) Tests:

• Testing for AgentRegistry is TBD.
• Testing for mTLS on AgentRegistry is done. See colab and https://screenshot.googleplex.com/4qqaYT54T97tM96

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.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Add any other context or screenshots about the feature request here.