chore: refactor CLI agent auth tests as unit tests (#19609)

Fixes https://github.com/coder/internal/issues/933

Refactors CLI tests that check the `--auth` flag parsing for various public clouds into a unit test that just creates the agent Client and asserts on the type.

Testing that the agent client actually authenticates correctly with these auth types is well covered by Coderd tests, so we don't need to retread that ground here, and the deleted tests were flaky on Windows.

cli/gitssh.go

@@ -39,7 +39,7 @@ func gitssh() *serpent.Command {
 				return err
 			}
 
-			client, err := agentAuth.CreateClient(ctx)
+			client, err := agentAuth.CreateClient()
 			if err != nil {
 				return xerrors.Errorf("create agent client: %w", err)
 			}