fix(dashboards): Default axisRange to auto for existing widgets in builder

[mtopo27]

When editing an existing widget that was saved before the axisRange feature (#109390), the builder showed the dataset default (e.g. "Fit to data" for mobile app size) as checked, even though the widget was actually rendering with auto (Y-axis starting at 0). This created a mismatch between the builder UI and the actual chart behavior.

The fix normalizes missing axisRange to 'auto' when loading an existing widget into the builder. This way undefined in builder state unambiguously means "new widget, apply dataset defaults" — existing widgets get an explicit value that matches their current rendering.

• Existing widgets: builder shows unchecked, matching actual rendering
• New widgets: dataset default still applies (e.g. dataMin for mobile app size)
• Existing widgets with explicit axisRange: no change

Refs EME-890

[linear]

EME-890 Add configurable Y-axis range setting for dashboard timeseries widgets

[cursor]

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

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[sentry]

Bug: New widgets created from templates incorrectly default to axisRange: 'auto', overriding dataset-specific defaults like 'dataMin' and causing incorrect Y-axis rendering.
_{Severity: MEDIUM}

Suggested Fix

Modify the logic to differentiate between editing an existing widget and creating a new one from a template. For new widgets from templates, axisRange should remain undefined in the builder state to allow the dataset-specific default to be applied during the final widget conversion. The 'auto' default should only apply to existing widgets being edited.

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/widgetBuilder/utils/convertWidgetToBuilderStateParams.ts#L79

Potential issue: The logic in `convertWidgetToBuilderStateParams` now sets `axisRange`
to `'auto'` if it's initially `undefined`. This affects new widgets created from
templates, as they don't have an `axisRange` property. Consequently, the builder state
becomes `axisRange: 'auto'`. When converting back, this `'auto'` value is preserved,
preventing the widget from inheriting the intended dataset-specific default axis range
(e.g., `'dataMin'`). This results in new widgets rendering with an incorrect Y-axis that
starts at 0 instead of the dataset's default.

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