Add JSON download modebar button

[gokul-debugger]

What does this PR do?

Adds an opt-in downloadJson modebar button for exporting the current Plotly figure as JSON.

The button can be enabled through:

config: {
    modeBarButtonsToAdd: ['downloadJson']
}

or through layout.modebar.add.

It uses Plotly's existing full-json export pipeline, preserving the established serialization behavior for figure data, layout, config, typed arrays, and version metadata. Download failures are reported through the existing Plotly notifier.

This replaces #7963, which GitHub closed after its base branch was deleted.

Closes #7917

Testing

• npm run test-jasmine -- modebar --nowatch (103 tests passed)
• npm run lint
• npm run typecheck
• npm run test-syntax
• npm run schema-typegen-diff-check
• git diff --check

[gokul-debugger]

All substantive CI checks are passing. The only failure is flaky-no-gl-jasmine, which appears unrelated to this change. I do not have permission to rerun it from my fork. Could a maintainer please rerun that check when convenient?

[camdecoster]

This looks good and is working in my testing. I'd like to move where the button gets added and add a success message for parity with the toImage button. Once that's done, I can approve and merge.

[camdecoster]

I think it would be more appropriate to keep this under 'Other'.

Suggested change

	| 'downloadJson'
	// Other
	// Other
	| 'downloadJson'

[camdecoster]

Let's add the success message along with the error message.

Suggested change

	modeBarButtons.downloadJson = {
	name: 'downloadJson',
	title: function (gd) {
	return _(gd, 'Download plot as JSON');
	},
	icon: Icons.disk,
	click: function (gd) {
	Registry.call('downloadImage', gd, {format: 'full-json'}).catch(function () {
	Lib.notifier(_(gd, 'Sorry, there was a problem downloading your JSON file!'), 'long', gd);
	});
	}
	};
	modeBarButtons.downloadJson = {
	name: 'downloadJson',
	title: (gd) => _(gd, 'Download plot as JSON'),
	icon: Icons.disk,
	click: (gd) => {
	Registry.call('downloadImage', gd, { format: 'full-json' })
	.then((filename) => {
	Lib.notifier(_(gd, 'JSON download succeeded') + ' - ' + filename, 'long', gd);
	})
	.catch(() => {
	Lib.notifier(_(gd, 'Sorry, there was a problem downloading your JSON file!'), 'long', gd);
	});
	}
	};

[camdecoster]

Let's move this section down and add a boolean to track if the button should be included.

Suggested change

	let addDownloadJson = false;

[camdecoster]

Suggested change

	newList.push(modeBarButtons.downloadJson);
	addDownloadJson = true;

[camdecoster]

By moving this down, we can place the downloadJson button next to the download image button, which makes more sense to me.

Suggested change

	// buttons common to all plot types
	const commonGroup = ['toImage'];
	if (addDownloadJson) commonGroup.push('downloadJson');
	if (context.showSendToCloud) commonGroup.push('sendChartToCloud');
	addGroup(commonGroup);
	addGroup(dragModeGroup);