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