Add provider error fixture corpus

[slegarraga]

Summary

• add public provider error fixtures for OpenAI, Anthropic, Gemini and transport failures
• pair each raw SDK-like/fetch-like case with expected normalized output
• publish fixtures in the npm package and bump to 0.1.4

Validation

• npm run format:check
• npm run typecheck
• npm run lint
• npm test
• npm run build
• npm audit --audit-level=moderate
• npm pack --dry-run --json verified 23 fixture files

Closes #1