Fix nested JSON parameter normalization

[adsharma]

Summary

Fix to_json($param) handling for Python JSON parameters that contain mixed scalar and nested values in the same list.

Root Cause

The JSON parameter normalization path only forced JSON serialization for empty or otherwise unresolved nested shapes. Lists like ["entry1", "entry2", {"key": "value"}] were treated as typed parameters even though their mixed scalar/object structure is a JSON shape.

Changes

• Detect mixed scalar and nested list elements as requiring JSON serialization.
• Add an integration regression for nested @context JSON data.
• Add a normalization regression that ensures this path does not import NumPy for non-NumPy users.

Validation

• uv run pytest test/test_json.py test/test_parameter.py test/test_issue.py::test_issue_483_numpy_ndarray_parameter -q
• uvx ruff check src_py/connection.py test/test_json.py