OAuth2 Token Exchange Fails for Providers Requiring client_secret_post Authentication

[asalauyeu-hubspot]

Describe the bug
ADK's OAuth2 credential exchanger fails to exchange authorization codes for access tokens when working with OAuth providers that require client_id and client_secret to be sent in the POST request body (using client_secret_post method) rather than via HTTP Basic Authentication.

Expected behavior
ADK should support OAuth providers that require different token endpoint authentication methods, particularly client_secret_post which is specified in RFC 6749 and commonly required by enterprise OAuth providers.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. macOS, Linux, Windows]
• Python version(python -V): 3.13
• ADK version(pip show google-adk): 1.17.0

Additional context
This is similar to:

• Support OAuth2 Token Endpoint Authentication Methods #2871
• oauth exchange code of token getting 400 BAD REQUEST as client id is required #2806