fix(auth): Support fallback OAuth token and prefixless credential lookups in session state

[tonycoco]

Fixes #4712

Session state might store authentication responses as raw string tokens instead of AuthCredential objects, or under custom credential keys without the standard "temp:" prefix.

Add robust fallback handling to resolve raw token strings, check for prefixless keys, and scan state values for any Google OAuth access tokens starting with "ya29."

The patch updates the get_auth_response method in google/adk/auth/auth_handler.py to do the following:

Check temp:<key> prefix: This maintains the standard OAuth redirection flow.
Check <key> directly: This allows resolving credentials where the application or environment provides them under the exact configuration key without prefixes.
Scan session state for OAuth tokens: This dynamically scans the state dictionary for active Google Cloud access tokens starting with the standard "ya29." prefix. This is common in hosted Google Cloud environments such as Vertex AI Reasoning Engine.

Unit Tests:

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

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.

[adk-bot]

Response from ADK Triaging Agent

Hello @tonycoco, thank you for creating this PR!

This PR is a bug fix for #4712, but we noticed that it is missing some information required by our contribution guidelines:

• Testing Plan: Please include a testing plan section in your PR description to describe how you tested or will test these changes.
• Logs or Screenshots: For bug fixes, please provide logs or a screenshot after the fix is applied.

Providing this information will help reviewers process and review your PR much more efficiently. Thank you for your contribution!