fix: write and read figure JSON as UTF-8

[LukeTheoJohnson]

Problem

pio.write_json(fig, path) raises UnicodeEncodeError for any figure containing non-ASCII text (titles, axis labels, hover text, categorical values such as °C, café, µ) on platforms whose default text encoding is not UTF-8. On Windows the default is cp1252:

fig.update_layout(title="Café µ")
pio.write_json(fig, "fig.json")
# UnicodeEncodeError: 'charmap' codec can't encode characters ...

read_json has the same bug. It reads the file without specifying an encoding, so a UTF-8 JSON file is decoded as cp1252 and the text comes back as mojibake. This works on macOS/Linux only because their default encoding happens to be UTF-8.

Root cause

plotly/io/_json.py opened the file without an encoding on both sides:

• write_json: path.write_text(json_str)
• read_json: path.read_text()

write_html already does this correctly (path.write_text(html_str, "utf-8")), and the same class of bug was previously fixed for HTML output (#3898). I believe the JSON I/O path was simply missed.

Fix

Pass "utf-8" explicitly in both write_json and read_json, matching write_html. JSON is UTF-8 by default per RFC 8259.

Tests

• Updated the existing test_write_json_pathlib and test_read_json_from_pathlib to assert the UTF-8 encoding is used. Verified red to green: both fail on the unpatched source and pass with the fix.
• Confirmed a real write_json to read_json return of a non-ASCII figure returns the original text on Windows (cp1252).
• No new regressions: tests/test_io/test_to_from_json.py shows only the pre-existing FigureWidget ImportErrors (anywidget not installed).

Scope

Small source change plus test assertions. No behaviour change on platforms that already default to UTF-8.