test: add coverage for null error object fallback path

Add test case for the scenario where API returns valid JSON but
the error object parses to null, triggering the final fallback
error message path in NylasClient.

This improves test coverage for the enhanced error handling
introduced in the previous commit.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: googsvg

src/test/kotlin/com/nylas/NylasClientTest.kt

@@ -734,5 +734,32 @@ class NylasClientTest {
       assertNotNull(exception.providerError)
       assertEquals("invalid_grant", exception.providerError!!["error"])
     }
+
+    @Test
+    fun `API error with valid JSON but null error object returns helpful fallback message`() {
+      // This tests the final fallback path when JSON parsing succeeds but returns null
+      val validJsonButNullError = """{"some_field": "some_value"}"""
+
+      val mockResponse = mock(okhttp3.Response::class.java)
+      val mockBody = mock(ResponseBody::class.java)
+
+      whenever(mockResponse.isSuccessful).thenReturn(false)
+      whenever(mockResponse.code).thenReturn(503)
+      whenever(mockResponse.body).thenReturn(mockBody)
+      whenever(mockBody.string()).thenReturn(validJsonButNullError)
+      whenever(mockResponse.headers).thenReturn(Headers.headersOf())
+      whenever(mockCall.execute()).thenReturn(mockResponse)
+
+      val client = NylasClient("test-api-key", mockOkHttpClientBuilder)
+
+      val exception = assertFailsWith<NylasApiError> {
+        client.grants().list()
+      }
+
+      assertEquals("unknown", exception.type)
+      assert(exception.message.contains("API request failed with status 503"))
+      assert(exception.message.contains("Response body:"))
+      assertEquals(503, exception.statusCode)
+    }
   }
 }