test: add callAnthropic tests with record/replay fixtures

[seba3c]

Summary

Add comprehensive tests for callAnthropic in lib/prompts.mjs with a record/replay fixture system for real API responses.

What's changed

• Removed /* v8 ignore start/end */ comments around callAnthropic so coverage tracks it
• Added lib/__tests__/call-anthropic.test.mjs with 13 tests:
  • Successful 200 response (using buildAuditPrompt with simple CSS)
  • Missing apiKey guard
  • All 10 documented Anthropic HTTP error codes (400–529)
  • Generic error fallback when response message is missing
• Added lib/__tests__/helpers/anthropic-recorder.mjs — lightweight fixture recorder
• Added lib/__fixtures__/anthropic/ directory for stored responses
• Added test:record npm script (RECORD_FIXTURES=1 vitest run)

Verification

• All 78 tests pass
• prompts.mjs coverage: 100% statements, 88.57% branches

[seba3c]

@nujovich I am not able to assign the PR to you as a reviewer, can u check this option?