Fix find_additional_properties ignoring an empty patternProperties key

[vineethsaivs]

find_additional_properties in jsonschema/_utils.py collapses every patternProperties key into one regex with "|".join(...) and then guards it with if patterns:

patterns = "|".join(schema.get("patternProperties", {}))
for property in instance:
    if property not in properties:
        if patterns and re.search(patterns, property):
            continue
        yield property

"|".join maps both an empty patternProperties and {"": ...} to the same empty string, and if patterns then treats that empty (match-everything) pattern as absent. So an empty-string pattern key, which is a valid regex that matches every property, is silently dropped: its properties are reported as additional and validated against additionalProperties instead of the pattern subschema.

from jsonschema import Draft202012Validator as D

# "" matches every property, so patternProperties owns "x"; nothing is additional.
list(D({"additionalProperties": False,
        "patternProperties": {"": {"type": "integer"}}}).iter_errors({"x": 1}))
# before: ["'x' does not match any of the regexes: ''"]   (should be valid)

# The wrong subschema is applied, not just a spurious message:
list(D({"patternProperties": {"": {"type": "string"}},
        "additionalProperties": {"type": "integer"}}).iter_errors({"x": "hi"}))
# before: ["'hi' is not of type 'integer'"]   (should be valid: "x" matches "" -> string subschema)

The sibling helper find_evaluated_property_keys_by_schema (used for unevaluatedProperties) already iterates the patterns individually with re.search, so the two helpers disagree on the same schema. This change makes find_additional_properties do the same: iterate the patternProperties dict and skip a property if any pattern matches, which distinguishes "no patterns" (nothing to match) from an empty-string pattern (matches everything).

Added TestFindAdditionalProperties in jsonschema/tests/test_utils.py (fails before, passes after). The full JSON-Schema-Test-Suite still passes with no regressions.

[fallenmi]

Verified with Codex at exact head 2bd25c718550fcd37809719d658247721a207989.

On the exact base, both externally visible cases are RED: an empty patternProperties key incorrectly makes additionalProperties: false reject {"x": 1}, and a value matched by that empty pattern is incorrectly validated by the additionalProperties subschema. At this head, both schemas produce zero errors.

The one-commit diff also applies cleanly to current main b37f7be6dc7966a1f1a67557976041ffe0826cb3. The focused utility suite is 40/40, and the full no-extras suite ran 8,514 tests on both the exact head and the current-main integration (7,811 successes, 703 skips, no failures). Iterating the patterns individually also aligns this helper with the sibling evaluated-properties path while correctly distinguishing no patterns from the valid empty-string regex.

[vineethsaivs]

Thank you for reproducing it independently, including running the full no-extras suite on both the head and a current-main integration.

To save a maintainer the re-check: the change is one commit touching find_additional_properties only, and it makes the helper iterate patternProperties individually so an empty pattern key stops being treated as "no patterns at all". That aligns it with the sibling evaluated-properties path, which already does this. Both externally visible symptoms are red on the base and green here: an empty patternProperties key made additionalProperties: false reject a perfectly valid instance, and a property matched by that empty pattern skipped the additionalProperties subschema entirely.

@Julian, happy to rebase or split this if you would like it in a different shape.