feat: add configurable box border style

[Manuel R. Sabella (manrods)]

Summary

• add a validated boxBorderStyle configuration option with square as the backward-compatible default
• support square (┌ ┐ └ ┘) and rounded (╭ ╮ ╰ ╯) corners across suggestion and description boxes
• keep renderBox pure by resolving configuration at the composition root and passing a typed, immutable style through the UI
• add coverage for TOML validation, defaults, box rendering, and style propagation

Design

BoxBorderStyle is derived from one runtime list shared with the AJV schema. The configuration loader returns a resolved configuration snapshot, while the renderer receives only the visual option it needs. Border glyphs remain private and exhaustively typed.

This avoids coupling rendering utilities to the global configuration singleton and keeps tests free of global-state mutation.

Testing

• npm run build
• npm run lint
• npm test -- --runInBand
  • 207 passed
  • 10 skipped
  • 93 snapshots passed

Closes #452

[Manuel R. Sabella (manrods)]

@microsoft-github-policy-service agree

[cpendery (cpendery)]

Hey Manuel R. Sabella (@manrods), I think your PR is more complicated than necessary, please take a look at https://github.com/microsoft/inshellisense/pull/454/changes

[Manuel R. Sabella (manrods)]

Noted! Working on a simpler solution.