fix: misleading error after OAuth is disabled (#299)

In the edge case where the user disables the OAuth authentication while
the authorization page is opened on the server, and then user hits
Allow, the URI executes. But the plugin tries to initialize the rest api
client and CLI without checking that OAuth is no longer allowed. This
raises a misleading error.

The fix is somewhat simple, before exchanging the authorization code
with an acces token we check if OAuth is still enabled and if not we
fail fast with a proper error message.
For cases where Cancel is selected by the user on the authorization page
we allow the flow to go as usual in Toolbox.

- raised coder/coder#24912 while investigating
this issue
- resolves https://linear.app/codercom/issue/DEVEX-222

Author: fioan89

src/main/kotlin/com/coder/toolbox/CoderRemoteProvider.kt

@@ -434,7 +434,14 @@ class CoderRemoteProvider(
             FAILED_TO_HANDLE_OAUTH2_TITLE,
             "OAuth2 server did not respond back with an access token"
         )
-
+        // before going forward we check to make sure OAuth is not disabled in the meantime
+        if (!context.settingsStore.preferOAuth2IfAvailable) {
+            context.logAndShowError(
+                FAILED_TO_HANDLE_OAUTH2_TITLE,
+                "OAuth based authentication is not enabled for Coder plugin in Toolbox. Please enable it in plugin settings or use the API token instead."
+            )
+            return
+        }
         exchangeOAuthCodeForToken(code, CoderSetupWizardContext.oauthSession!!)
     }