feat(api): add configurable AI models and improve error messaging

[nandes007]

Description of Changes

1. Root Cause / Problem Addressed

• Hardcoded & Deprecated Model IDs: Previously, groqClient used hardcoded models and geminiClient had a hardcoded endpoint (gemini-2.5-flash). When model availability changes or when models are deprecated/inaccessible on a user tier, requests fail with 404 Not Found.
• Vague & Masked Error Messages: Axios errors were caught by reading only error.message (which outputs generic strings like Request failed with status code 404), completely discarding the detailed error payload from Groq/Gemini (error.response.data.error.message).

2. Summary of Adjustments

• Configurable Models:
  • Added GROQ_MODEL (defaults to openai/gpt-oss-120b) and GEMINI_MODEL (defaults to gemini-3.6-flash) in src/config.ts.
  • Added support for configuring models via malas setConfig <KEY> <VALUE>, environment variables (GROQ_MODEL, GEMINI_MODEL), and CLI flags (-m, --model) on malas, malas generate, and malas pr.
• Human-Readable Error Messages:
  • Enhanced error handling in groqClient.ts and geminiClient.ts using axios.isAxiosError to extract the actual API error message.
  • Formatted specific, actionable error messages for HTTP 401 (invalid API key), 404 (model not found/deprecated with command suggestions), and 429 (rate limit/quota exceeded).
  • Cleaned up error cascades and fallback messaging in aiClient.ts and commitService.ts.
• Test Suite:
  • Added tests/aiClients.test.ts to test client error handling, status code mappings, and model parameters.
  • Updated tests/commitService.test.ts to align with the current prompt and test model propagation.

Related Issues

N/A

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation update

Testing Information

• Ran unit tests with Jest: npm test (11/11 tests passing across 2 test suites).
• Built project: npm run build (TypeScript compilation and import extension fixes).
• Verified live end-to-end completions with active Groq and Gemini APIs.