fix(ui): use variation name as label in evaluation chart for YAML/JSON type

[steveninhle]

Summary

• Evaluation chart legend was showing raw YAML/JSON blob content as the
  variation label, making it unreadable for those variation types
• Fix: resolve variation label using variation.name || variation.value,
  which prefers the human-readable name when set and falls back to value
  otherwise
• This aligns the chart legend with the evaluation table, which already
  uses the same name || value fallback pattern

[Copilot]

Pull request overview

Updates the feature-flag evaluation chart legend to display human-readable variation labels for JSON/YAML (and other) variation types by preferring a variation’s name and falling back to value, matching the behavior already used in the evaluation table.

Changes:

• Resolve each chart legend label via variation.name || variation.value (with fallback to "default value" for the default bucket).
• Refactor label resolution to avoid repeating the find call inline and improve readability.

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