test: add coverage for chartToTable (lib/tools/chart-util.ts) - #35

Summary

Adds tests/chart-util.test.ts with a dedicated unit test suite for chartToTable in src/lib/tools/chart-util.ts (resolving #24).

Tests Covered

• Pie charts (type: "pie"):

  • Standard pie chart spec conversion with name/value columns and mapped rows.
  • Pie chart specs with missing or empty data arrays.

• Cartesian charts (type: "bar" / type: "line"):

  • Bar charts with xLabels and multiple series.
  • Line charts with single or multiple series.
  • Fallback to 1-indexed string row numbers ("1", "2", ...) when xLabels is missing.
  • Edge case: Empty series array.
  • Edge case: Shorter series data array (pads missing values with null).
  • Edge case: Longer series data array (asserts current behavior that rows walk series[0].data).

Validation

• bun run test chart-util — 8 tests passed
• bun run typecheck — clean
• bun run lint — clean
• bun run test — 304 tests passed across 28 test files

Closes #24