🔧 feat: Add Basic Token Exchange Method for Actions OAuth flow

[danny-avila]

Summary

Closes #7840

I added support for handling a token_exchange_method parameter to enable OAuth token exchanges using either default POST or HTTP Basic authentication, enhancing the Actions OAuth flow.

• Extended the Actions OAuth callback and creation logic to pass and store the token_exchange_method parameter.
• Updated the TokenService to select the token exchange mechanism based on the new parameter, supporting both default_post and basic_auth_header strategies.
• Modified access and refresh token retrieval to conditionally move client credentials to the basic auth header, as required by the token_exchange_method.
• Improved documentation and parameter validation for new OAuth exchange options.

Change Type

• New feature (non-breaking change which adds functionality)
• Documentation update

Testing

I tested this by registering actions with both token exchange methods, completing the OAuth flow, and verifying correct token storage and response.

To further validate, I recommend:

• Setting up a mock OAuth server supporting both token exchange patterns.
• Executing OAuth actions with each method and confirming token issuance in the expected format.
• Reviewing server logs to ensure the chosen exchange method is used correctly.
• Adding unit tests for the TokenService covering both exchange paths.

Test Configuration:

• Manual endpoint testing with OAuth providers
• Local environment with dummy secrets
• librechat-data-provider dependency providing required enums

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes