fix: added support for percentFormat in Segment Label

[Natansh271828]

Description

Added a new percentFormat prop to the SegmentLabel component that allows customizing the d3 format specifier used for percentage labels on donut chart segments. Previously, the percentage format was hardcoded to '.0%' (whole percentages like "45%"), making it impossible to display decimal percentages (e.g., "45.3%"). This also caused precision loss through rounding — for example, values like 0.4% and 1.2% would both round to "0%" and "1%" respectively, making it difficult to distinguish between small but meaningfully different segments.
The change spans both the main (vega-spec-builder) and S2 (vega-spec-builder-s2) packages, keeping them in sync.

Related Issue

N/A — discovered while consuming the library in a downstream project where valueFormat was being passed with percent: true, but had no effect on the percentage display.

Motivation and Context

When using , there was no way to control the precision of the percentage label. The format was hardcoded to '.0%' in getSegmentLabelValueText(). The existing valueFormat prop only applies to the metric value (when value: true), not to the percentage — which was not obvious from the API surface. Adding percentFormat gives consumers explicit control over percentage formatting while maintaining backward compatibility with the '.0%' default.

How Has This Been Tested?

Screenshots (if appropriate):

Added 4 new unit tests (2 per spec-builder package) covering:
Custom percentFormat with percent: true only
Custom percentFormat in combined percent + value mode
All 39 existing tests continue to pass (yarn test -- --testPathPattern="segmentLabelUtils")
Locally tested with a downstream consumer app via pnpm link to verify the percentage labels render with the custom format

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[c-lamoureux]

Changes look great. Can we add the prop and description to the Docs package?