fix(dashboards): Persist legend selection to URL for new chart widgets

[gggritso]

Wire widgetLegendState through to VisualizationWidget so that toggling series visibility in the legend of new TimeSeriesWidgetVisualization charts persists to the unselectedSeries URL parameter. Previously, legend selections were lost on page refresh.

The new chart path uses plain series names internally, while WidgetLegendSelectionState uses seriesName|~|widgetId-encoded keys for per-widget disambiguation (needed because all dashboard charts share an ECharts chart group). This PR adds encode/decode at the VisualizationWidget boundary to bridge the two formats.

Also removes a now-unnecessary decode call from formatTimeSeriesLabel — the yAxis field is never encoded with |~|widgetId in the new path, so the decode was dead code. Corresponding test cases for encoded yAxis values are removed.

Closes BROWSE-199

[linear-code]

BROWSE-199 New charts do not update URL with legend selection

[sentry]

Sentry Snapshot Testing

Name	Added	Removed	Modified	Renamed	Unchanged	Status
sentry-frontend sentry-frontend	0	0	0	0	204	✅ Unchanged

[sentry]

Bug: encodeLegendSelection is called with an undefined widget.id for unsaved widgets, creating invalid legend keys. Downstream decoding logic using a non-null assertion on widget.id will then fail.
_{Severity: LOW}

Suggested Fix

Add explicit validation in widgetCanUseTimeSeriesVisualization to ensure a widget has an ID before allowing legend state changes. Alternatively, defensively handle undefined IDs in the encoder and decoder functions to prevent creating invalid keys.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: static/app/views/dashboards/widgetCard/visualizationWidget.tsx#L537-L541

Potential issue: The `Widget` type allows `widget.id` to be optional. The function
`widgetCanUseTimeSeriesVisualization` determines if a widget can use the new time series
visualization but does not validate that `widget.id` is present. If a widget without an
ID (e.g., an unsaved widget) meets the criteria, `encodeLegendSelection` is called with
an `undefined` ID. This creates an invalid legend key containing the literal string
"undefined", such as `"seriesName|~|undefined"`. Downstream decoding logic in
`widgetLegendSelectionState.tsx` uses a non-null assertion (`widget.id!`), which will
fail or cause incorrect behavior when it encounters these malformed keys.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Removing decode also removes function name prettification

Low Severity

The removed decodeSeriesNameForLegend call did two things: strip the |~|widgetId suffix (correctly identified as dead code) and prettify aggregate function names via prettifyParsedFunction (e.g., count(span.duration) → count(spans)). By removing the entire call, the prettification side-effect is also lost, causing a subtle label regression in the new chart path for specific aggregates like count(span.duration).

 

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