Fix: Extract non-JSON error body (#33)

Before:
When the inference service returns a non-JSON response, the actual error
is masked by a parsing failure.
```text
Caused by: java.lang.IllegalArgumentException: Could not process JSON error object
Caused by: com.fasterxml.jackson.core.JsonParseException:
Unrecognized token 'dial'
```

After:
The original error is preserved.
```text
Caused by: com.bakdata.kserve.client.InferenceRequestException:
Inference request failed: 502:
dial tcp 127.0.0.1:8080: connect: connection refused
```

---------

Co-authored-by: Philipp Schirmer <philipp.schirmer@bakdata.com>

Author: vostres

src/main/java/com/bakdata/kserve/client/KServeClientV2.java

@@ -55,7 +55,8 @@ protected String extractErrorMessage(final String stringBody) {
                     .or(() -> Optional.ofNullable(inferenceError.getDetail()))
                     .orElseThrow(() -> new InferenceRequestException("Could not extract error message."));
         } catch (final JsonProcessingException e) {
-            throw new IllegalArgumentException("Could not process JSON error object", e);
+            log.warn("Could not parse error body as JSON: {}", stringBody, e);
+            return stringBody;
         }
     }
 

src/test/java/com/bakdata/kserve/client/KServeClientV2Test.java

@@ -137,6 +137,34 @@ public MockResponse dispatch(@NotNull final RecordedRequest recordedRequest) {
                 .hasMessage("Inference request failed: 400: Not Found");
     }
 
+    @Test
+    void testFallbackNonJsonBody() {
+        final Dispatcher dispatcher = new Dispatcher() {
+            @NotNull
+            @Override
+            public MockResponse dispatch(@NotNull final RecordedRequest recordedRequest) {
+                return new MockResponse.Builder()
+                        .code(404)
+                        .body("Not found")
+                        .build();
+            }
+        };
+        this.mockServer.getMockWebServer().setDispatcher(dispatcher);
+
+        final KServeClientV2<String> client = KServeClientV2.<String>builder()
+                .serviceBaseUrl(this.mockServer.getServiceBaseUrl())
+                .modelName("test-model")
+                .httpClient(KServeClient.getHttpClient(Duration.ofMillis(10000)))
+                .build();
+
+        final InferenceRequest<String> fakeInferenceRequest = getFakeInferenceRequest("data");
+
+        this.softly.assertThatThrownBy(
+                        () -> client.makeInferenceRequest(fakeInferenceRequest, FakePrediction.class, ""))
+                .isInstanceOf(InferenceRequestException.class)
+                .hasMessageContaining("Not found");
+    }
+
     @Test
     void testRetry() throws IOException {
         this.mockServer.setUpForRetryTest();