Serialize figures without deep copying them

[T4rk1n]

Link to issue

Reported in plotly/dash#4065. The Dash side is plotly/dash#4067.

Description of change

to_json_plotly and PlotlyJSONEncoder no longer deep copy figures, and with orjson only the values orjson can't serialize go through clean_to_json_compatible.

• BaseFigure._to_json_dict() and BasePlotlyType._to_json_dict() return the object's data without the deepcopy that to_dict() / to_plotly_json() do. Typed arrays are converted with a new to_base64_copy, which copies only the containers it changes, so the figure is never mutated. Subclasses that override to_dict or to_plotly_json still go through their override.
• The orjson path passes default= to orjson.dumps, so one non-native value (a go.Figure, a pd.Timestamp) no longer sends the whole payload through the Python walk. Anything that still fails takes the old path, so errors are unchanged.
• PlotlyJSONEncoder.default uses _to_json_dict and returns isoformat() for plain datetime / date. Subclasses keep the old method order, so their encode_as_* overrides still apply.

Demo

10 traces x 20k points, to_json_plotly median ms, before -> after:

data	value	orjson	json
numpy array	go.Figure	5.3 -> 5.0	13.2 -> 12.7
list of float	go.Figure	36 -> 5.8	82 -> 53
list of float	{"f": go.Figure, "t": pd.Timestamp}	61 -> 5.9	82 -> 53
pd.DatetimeIndex	{"f": go.Figure, "t": pd.Timestamp}	41 -> 11	74 -> 74
list of datetime	go.Figure	179 -> 10.5	496 -> 125
list of datetime	{"f": go.Figure, "t": pd.Timestamp}	304 -> 10	490 -> 126

Output is byte-identical to main on both engines, pretty and compact, for px figures, frames, templates, geojson, tuples, numpy/pandas/datetime/Decimal/NaN values, and mixed dicts. Known differences, both with orjson:

• A raw numpy datetime64 array sitting next to a value orjson can't serialize now uses orjson's form (2024-01-01T05:00:00) instead of datetime_as_string (2024-01-01T05:00:00.000000000). That is the same output you already get when the array is on its own.
• Some payloads that used to raise TypeError: Type is not JSON serializable: NaTType now serialize, as they already did with the json engine.

Testing strategy

• test_figure_is_not_deep_copied (all engines) fails serialization if deepcopy is called, checks the output against to_dict(), and checks that the figure's numpy arrays are left as they were.
• test_to_base64_copy_leaves_input_untouched covers the copy-on-write helper.
• test_subclass_overrides_are_kept and test_encoder_subclass_overrides_are_kept cover subclasses that override to_dict / to_plotly_json and the encoder's encode_as_* methods.

tests/test_io, tests/test_core, tests/test_plotly_utils and tests/test_optional pass locally, apart from failures that also happen on main here (test_trendline_on_timeseries[pyarrow-*] and a colorscale test that depends on test order).

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
• I have added an entry to the changelog if needed (not required for documentation PRs).