fix(antigravity): remove credits fallback from CountTokens, fix gofmt

CountTokens upstream API does not support enabledCreditTypes, so
remove the dead credits fallback path from ExecuteCount and delete
the unused tryAntigravityCreditsExecuteCount method. Fix gofmt on
credits test file.

internal/runtime/executor/antigravity_executor_credits_test.go

@@ -127,7 +127,6 @@ func TestAntigravityShouldRetryNoCapacity_Standard503(t *testing.T) {
 	}
 }
 
-
 func TestInjectEnabledCreditTypes(t *testing.T) {
 	body := []byte(`{"model":"claude-sonnet-4-6","request":{}}`)
 	got := injectEnabledCreditTypes(body)
@@ -158,7 +157,6 @@ func TestParseRetryDelay_HumanReadableDuration(t *testing.T) {
 	}
 }
 
-
 func TestAntigravityExecute_RetriesTransient429ResourceExhausted(t *testing.T) {
 	resetAntigravityCreditsRetryState()
 	t.Cleanup(resetAntigravityCreditsRetryState)