Add unit test to ensure failures don't tank the workflow run

Actually this time.

tests/langkit/callbacks/test_webhook.py

@@ -0,0 +1,26 @@
+from langkit.callbacks.library import lib as callback_lib
+from langkit.core.validation import ValidationFailure
+from langkit.core.workflow import EvaluationWorkflow
+from langkit.metrics.library import lib as metric_lib
+from langkit.validators.library import lib as validator_lib
+
+
+def test_webhook_failures_dont_ruin_run():
+    wf = EvaluationWorkflow(
+        metrics=[metric_lib.text_stat.char_count.prompt],
+        validators=[validator_lib.constraint("prompt.char_count", upper_threshold=5)],
+        callbacks=[callback_lib.webhook.basic_validation_failure("https://foo.bar")],  # will fail, url doesn't exist
+    )
+
+    result = wf.run({"prompt": "1234567890"})
+
+    assert result.validation_results.report == [
+        ValidationFailure(
+            id="0",
+            metric="prompt.char_count",
+            details="Value 10 is above threshold 5",
+            value=10,
+            upper_threshold=5,
+        )
+    ]
+    assert result.metrics["prompt.char_count"][0] == 10