Add missing return type annotation to json_extension.dump()

[Copilot]

The dump() function wrapper was missing a return type annotation, causing type checking ambiguity.

Changes

• Added -> None return type annotation to dump() in gridappsd/json_extension.py

The function wraps json.dump(), which writes to a file object and returns None. The annotation now matches the standard library's signature.

def dump(
    data: Any,
    fo: TextIO,
    *,
    skipkeys=False,
    ensure_ascii=True,
    check_circular=True,
    allow_nan=True,
    indent=None,
    separators=None,
    default=None,
    sort_keys=False,
    **kw,
) -> None:  # Added
    _json.dump(...)

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