feat(oidc-provider): allow passing oauth consent code via query params (#3845)

* feat: pass oauth consent code via query params

* address cubic comments

* fix tests

* address comments

docs/content/docs/plugins/oidc-provider.mdx

@@ -264,17 +264,40 @@ export const auth = betterAuth({
 })
 ```
 
-The plugin will redirect the user to the specified path with a `client_id` and `scope` query parameter. You can use this information to display a custom consent screen. Once the user consents, you can call `oauth2.consent` to complete the authorization.
+The plugin will redirect the user to the specified path with `consent_code`, `client_id` and `scope` query parameters. You can use this information to display a custom consent screen. Once the user consents, you can call `oauth2.consent` to complete the authorization.
 
 <Endpoint path="/oauth2/consent" method="POST" />
 
-```ts title="server.ts"
+The consent endpoint supports two methods for passing the consent code:
+
+**Method 1: URL Parameter**
+```ts title="consent-page.ts"
+// Get the consent code from the URL
+const params = new URLSearchParams(window.location.search);
+
+// Submit consent with the code in the request body
+const consentCode = params.get('consent_code');
+if (!consentCode) {
+	throw new Error('Consent code not found in URL parameters');
+}
+
 const res = await client.oauth2.consent({
 	accept: true, // or false to deny
+	consent_code: consentCode,
 });
 ```
 
-The `client_id` and other necessary information are stored in the browser cookie, so you don't need to pass them in the request. If they don't exist in the cookie, the consent method will return an error.
+**Method 2: Cookie-Based**
+```ts title="consent-page.ts"
+// The consent code is automatically stored in a signed cookie
+// Just submit the consent decision
+const res = await client.oauth2.consent({
+	accept: true, // or false to deny
+	// consent_code not needed when using cookie-based flow
+});
+```
+
+Both methods are fully supported. The URL parameter method works well with mobile apps and third-party contexts, while the cookie-based method provides a simpler implementation for web applications.
 
 ### Handling Login