Add test for JSON decode error handling in long documents with early errors.

tests/test_json_error_improvements.py

@@ -121,3 +121,21 @@ def test_json_decode_error_unclosed_brace():
     assert "column" in error["msg"].lower()
     assert error["type"] == "json_invalid"
     assert "position" in error["ctx"]
+
+
+def test_json_decode_error_in_middle_of_long_document():
+    # Create a JSON with error early in a long document (need >40 chars after error)
+    # The error is at position for "invalid" which needs at least 41 chars after it
+    long_json = '{"field": invalid, "padding_field": "this needs to be long enough that we have more than forty characters after the error position"}'
+
+    response = client.post(
+        "/items/", content=long_json, headers={"Content-Type": "application/json"}
+    )
+
+    assert response.status_code == 422
+    error = response.json()["detail"][0]
+
+    # The error snippet should have "..." at the end since error is early in doc
+    assert error["input"].endswith("...")
+    assert "invalid" in error["input"]
+    assert error["type"] == "json_invalid"