feat: Integrate mTLS support into Agent Registry API calls

[agrawalradhika-cell]

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 TBD.

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.

[adk-bot]

Response from ADK Triaging Agent

Hello @agrawalradhika-cell, thank you for creating this PR!

While reviewing your PR, I noticed a few items from our contribution guidelines are currently missing or incomplete:

1. Pytest Results Summary: Under the Unit Tests section, please include a summary of the passed pytest results.
2. Manual End-to-End (E2E) Tests: The testing plan currently lists the manual E2E tests for AgentRegistry and mTLS as "TBD". To maintain code quality and prevent regressions, please perform these tests and provide verifiable E2E evidence (such as console logs showing the successful mTLS session configuration/API requests).

Providing this information will help our reviewers review your PR more efficiently. Thank you!