fix(oauth): conditionally set client_id based on auth method

[imdominicreed]

Move client_id setting from generic request setup to auth-method-specific handlers. This ensures client_id is only included when using client assertion authentication, not for all request types.

Proposed Changes

Checklist

• I have added or updated unit tests
• I have added or updated integration tests (if appropriate)
• I have added or updated documentation

Testing Instructions

[gemini-code-assist]

Summary of Changes

Hello @imdominicreed, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the handling of the client_id parameter within OAuth requests. The change ensures that client_id is only included in the request form data when it is relevant to the specific authentication method being used, primarily for client assertion authentication. This prevents its unnecessary inclusion in other grant types, aligning the implementation more closely with OAuth specifications and improving the precision of authentication flows.

Highlights

• Conditional Client ID Setting: The client_id parameter is now set conditionally based on the OAuth authentication method, ensuring it is only included in requests when specifically required, such as for client assertion authentication.
• Client Credentials Grant Type: The client_id is no longer automatically included in the form data for requests using the client_credentials grant type.
• Password Grant Type (Certificate Exchange): The client_id has been removed from the initial form data setup for requests related to certificate exchange using the password grant type.
• Client Assertion Authentication: The client_id is now explicitly added to the form data when the authentication method is ClientAuthTypeClientAssertion.

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Client ID now waits, Only when assertion calls, Auth flow is precise.

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[gemini-code-assist]

Code Review

This pull request correctly refactors the OAuth client authentication logic to conditionally include the client_id parameter based on the authentication method. The changes ensure client_id is only sent in the request body for client assertion authentication, which aligns with OAuth 2.0 standards.

While the changes for the client_credentials grant in getAccessTokenRequest are implemented correctly, the modification in getCertExchangeRequest exposes a latent critical bug. The request body is constructed before the client authentication parameters are added, which will cause authentication to fail when using client assertions. I've provided a detailed comment with a suggested fix. Please note that getTokenExchangeRequest suffers from the same issue and should also be corrected.

[gemini-code-assist]

While removing the unconditional client_id here is correct, it reveals a critical logic bug in this function's order of operations.

The request body is created from data on line 342, but setClientAuth—which should conditionally add client_id for client assertion authentication—is called on line 355, after the body has been created. This means any modifications setClientAuth makes to the data map will not be included in the request body, breaking client assertion authentication.

To fix this, this function should be refactored to follow the pattern in getAccessTokenRequest:

1. Create the http.Request with a nil body.
2. Call setClientAuth to populate headers and/or the url.Values map.
3. Encode the url.Values and set the req.Body.

A similar bug also exists in getTokenExchangeRequest and should be addressed.