fix: resolve infinite hang when formatter binary is not installed

[stevan-borus]

Description

When formatter: 'prettier' or formatter: 'biome' is configured but the binary is not installed (e.g. after migrating to a different formatter), the plugin's buildStart hook hangs indefinitely. The Vite dev server never starts — no error, no timeout, just a silent hang.

This was introduced in #27 which moved formatter execution from direct API imports to tinyexec child processes to make formatters optional (#19). However, the error path wasn't wired up correctly.

Root cause: In lintFileContent(), when the formatter binary isn't found, tinyexec spawns a process that emits an error event (ENOENT). The error handler was empty and the promise only resolved on exit, which Node does not emit after an ENOENT spawn error — so the promise hung forever.

Fix: Wire the error handler to resolve the promise with the original (unformatted) content and log a warning, using a settled guard to prevent double-resolution when both error and exit fire.

Type of change

• 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 not work as expected)
• This change requires a documentation update
• Algorithm update - updates algorithm documentation/questions/answers etc.
• Other (please describe):

How Has This Been Tested?

• Integration tests
• Unit tests
• Manual tests
• No tests required

Unit test: Added a test that spawns a non-existent binary via tinyexec and asserts the promise resolves (within 5s) with the original content instead of hanging.

Manual test steps:

1. Configure iconsSpritesheet({ ..., formatter: 'prettier' }) in vite.config.ts
2. Ensure prettier is not installed in the project
3. Run vite dev
4. Before fix: dev server hangs indefinitely with zero output
5. After fix: warning logged ([icons-spritesheet] formatter "prettier" could not be started: spawn prettier ENOENT), spritesheet generated without formatting, dev server starts normally

Reviewer checklist

• Check if the UI is working as expected and is satisfactory
• Check if the code is well documented
• Check if the behavior is what is expected
• Check if the code is well tested
• Check if the code is readable and well formatted
• Additional checks (document below if any)

Screenshots (if appropriate):

N/A — this is a CLI/build-time fix with no UI changes.

Questions (if appropriate):

N/A