Convert string titles in updatemenu and slider args to title.text

[bhaskargurram-ai]

Link to issue

Closes #5032

Likely also addresses #5060 (same symptom: the title disappears after clicking a button since 6.0).

Description of change

Since plotly.js 3 (bundled from plotly.py 6.0), passing a string to a title attribute is no longer supported (plotly/plotly.js#7212). Figure properties are fine because TitleValidator maps strings to title.text, but the args of updatemenu buttons and slider steps are passed to Plotly.relayout/restyle/update verbatim, so a button with e.g. {"xaxis.title": "Time"} or {"xaxis": {"title": "Time"}} makes the title disappear.

This PR adds a MethodArgsValidator (used for layout.updatemenu.button.args, args2 and layout.slider.step.args via CUSTOM_VALIDATOR_DATATYPES in codegen) which converts plain string/number title values in these args:

• top-level attribute paths: {"xaxis.title": "Time"} -> {"xaxis.title.text": "Time"}. Like plotly.js < 3 did, this only updates the text and keeps other title properties such as the font.
• nested containers: {"xaxis": {"title": "Time"}} -> {"xaxis": {"title": {"text": "Time"}}}

Values that are already objects, title.text keys, other string properties and non-dict args (e.g. animation frame names) are left untouched, and the user's dicts are not mutated.

Demo

Clicking the buttons of a figure built with plotly.py, rendered with the bundled plotly.js 4.1.1:

	before	after
button relayout {"title": "Dotted", "xaxis.title": "x dotted", "yaxis.title": "y dotted"}	title, x and y titles disappear	"Dotted" / "x dotted" / "y dotted" (x title keeps its font)
button update [..., {"xaxis": {"title": "x nested"}, ...}]	titles disappear	"x nested" / "y nested"
slider step relayout {"xaxis.title": "x slider"}	title disappears	"x slider"

Testing strategy

Added tests/test_plotly_utils/validators/test_methodargs_validator.py (validator unit tests: coerced forms, forms left unchanged, no input mutation) and tests/test_core/test_graph_objs/test_method_args.py (serialized figure JSON for updatemenu args/args2, update args and slider steps). The string-title cases fail on main and pass with this change; existing validator/graph_objs/figure_messages tests are unchanged. plotly/validators/_validators.json was regenerated from codegen.

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).