cody-gateway: handle missing Google response (#63895)

- Logs a warning instead of returning an error when the Google response
is missing
- This prevents the API from returning an error when the Google response
is empty, which can happen when Google is not happy with the question
due to safety issues. We will only log a decoder error as error.

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## Test plan

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only the log level was updated for empty responses. the function
behavior was not changed.

## Changelog

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cody-gateway: log missing Google response as warning

cmd/cody-gateway/internal/httpapi/completions/google.go

@@ -168,5 +168,6 @@ func parseGoogleTokenUsage(r io.Reader, logger log.Logger) (promptTokens int, co
 		return res.UsageMetadata.PromptTokenCount, res.UsageMetadata.CompletionTokenCount, nil
 	}
 
-	return -1, -1, errors.New("no Google response found")
+	logger.Warn("no Google response found", log.Error(err))
+	return -1, -1, nil
 }

cmd/cody-gateway/internal/httpapi/completions/google_test.go

@@ -128,13 +128,13 @@ func TestParseGoogleTokenUsage(t *testing.T) {
 			name:    "no prefix",
 			input:   `{"usageMetadata": {"promptTokenCount": 10, "candidatesTokenCount": 20}}`,
 			want:    nil,
-			wantErr: true,
+			wantErr: false,
 		},
 		{
 			name:    "empty input",
 			input:   ``,
 			want:    nil,
-			wantErr: true,
+			wantErr: false,
 		},
 		{
 			name: "multiple lines with one valid",