fix(highcharts): keep tooltip formatter override stable across re-renders

[korvin89]

Problem

A custom libraryConfig.tooltip.formatter stops working after the first render. It became reliably reproducible after upgrading React 17 → 19: React 18+ StrictMode double-mounts components in dev (mount → unmount → remount), and the override silently falls back to the default ChartKit tooltip.

Root cause

prepareConfig was mutating the caller's libraryConfig:

params.tooltip.formatter = function (tooltip) {
    return `<div class="${b()}">${formatter.call(this, tooltip)}</div>`;
};
delete options.highcharts.tooltip.formatter; // ← deletes from the user's libraryConfig

options.highcharts is the consumer's libraryConfig (passed by reference, no clone). The wrapping works once, but the delete strips tooltip.formatter off the shared object. Any later config build that reuses the same libraryConfig reference — StrictMode double-mount, splitTooltip toggle, a data update reusing the same config, a second chart — no longer finds the formatter and renders the default tooltip.

It's a latent impurity in getDerivedStateFromProps; React 18+ StrictMode just surfaced it (exactly what it's designed to do). The StrictMode trigger is dev-only, but production triggers (data updates / splitTooltip / shared config) hit the same bug.

Fix

Don't mutate the input. The raw formatter is removed from the clone (preparedHighchartsOptions, which merge({}, …) already creates) right before the final mergeWith, so the wrapped formatter on params survives and libraryConfig is left untouched.

[gravity-ui-bot]

Preview is ready.

[korvin89]

/update-screenshots

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	18.64%	286 / 1534
🔵	Statements	18.73%	296 / 1580
🔵	Functions	14.5%	48 / 331
🔵	Branches	12.95%	149 / 1150

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[gravity-ui-bot]

🎭 Tests Report is ready.
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[korvin89]

Falling tests will be fixed here #895, and they are not related to current changes