fix: Match front and backend prompt variable behavior (#8522)

* fix handling of braces in front and back end

* add tests

* ruff check fix

* [autofix.ci] apply automated fixes

* [autofix.ci] apply automated fixes (attempt 2/3)

* [autofix.ci] apply automated fixes

* address pr comments and switches to use formatter

* remove code fence that isn't working

* comment

* [autofix.ci] apply automated fixes

* text fix

* style fix

* test fix

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/backend/tests/unit/utils/test_interface_utils.py

@@ -0,0 +1,60 @@
+import pytest
+from langflow.interface.utils import extract_input_variables_from_prompt
+
+
+@pytest.mark.parametrize(
+    ("prompt", "expected"),
+    [
+        # Basic variable extraction
+        ("Hello {name}!", ["name"]),
+        ("Hi {name}, you are {age} years old", ["name", "age"]),
+        # Empty prompt
+        ("", []),
+        ("No variables here", []),
+        # Duplicate variables
+        ("Hello {name}! How are you {name}?", ["name"]),
+        # Whitespace handling - Formatter preserves whitespace in field names
+        ("Hello { name }!", [" name "]),
+        ("Hi {  name  }, bye", ["  name  "]),
+        # Multiple braces (escaping)
+        ("Escaped {{not_a_var}}", []),
+        ("Mixed {{escaped}} and {real_var}", ["real_var"]),
+        ("Double escaped {{{{not_this}}}}", []),
+        # Complex cases
+        ("Hello {name}! Your score is {{4 + 5}}, age: {age}", ["name", "age"]),
+        ("Nested {{obj['key']}} with {normal_var}", ["normal_var"]),
+        ("Template {{user.name}} with {id} and {type}", ["id", "type"]),
+        # Edge cases
+        ("{single}", ["single"]),
+        ("{{double}}", []),
+        ("{{{}}}", []),
+        # Multiple variables with various spacing
+        (
+            """
+        Multi-line with {var1}
+        and {var2} plus
+        {var3} at the end
+        """,
+            ["var1", "var2", "var3"],
+        ),
+    ],
+)
+def test_extract_input_variables(prompt, expected):
+    """Test the extract_input_variables_from_prompt function with various cases."""
+    result = extract_input_variables_from_prompt(prompt)
+    assert sorted(result) == sorted(expected), f"Failed for prompt: {prompt}"
+
+
+@pytest.mark.parametrize(
+    ("prompt", "expected_error"),
+    [
+        # Malformed format strings that should raise ValueError
+        ("}{", "Single '}' encountered in format string"),
+        ("{incomplete", "expected '}' before end of string"),
+        ("incomplete}", "Single '}' encountered in format string"),
+    ],
+)
+def test_extract_input_variables_malformed(prompt, expected_error):
+    """Test that malformed format strings raise ValueError."""
+    with pytest.raises(ValueError, match=expected_error):
+        extract_input_variables_from_prompt(prompt)