feat: Implement multi-LLM provider support

[jivanandham]

This commit introduces a flexible architecture to support multiple LLM providers for problem analysis and solution generation.

Key changes:

• LLM Service Abstraction:
  • Introduced an LLMService interface (electron/llmService/llmServiceInterface.ts) defining a common contract for LLM operations (extractProblemInfo, generateSolution, debugSolution).
  • Refactored electron/handlers/problemHandler.ts to use a factory function (getLLMService) that dynamically instantiates an LLM service based on your preference.
• OpenAI Provider:
  • Created OpenAILLMService (electron/llmProviders/openai.ts) by migrating existing OpenAI API logic into this dedicated class.
• Provider Configuration:
  • Updated electron/store.ts to store API keys for OpenAI, Gemini (placeholder), Claude (placeholder), Local LLM, and GitHub Marketplace LLM (placeholder), along with a preferredProvider setting.
  • Revamped src/components/ApiKeyAuth.tsx to allow you to input API keys/configurations for multiple providers and select your preferred one. UI elements for Local LLM (base URL) and GitHub Marketplace LLM (API key, model ID) are included.
• Stubbed Providers:
  • Added LocalLLMService (electron/llmProviders/localLlm.ts) and GitHubMarketplaceLLMService (electron/llmProviders/githubMarketplaceLlm.ts) with stubbed methods as placeholders for future implementation.
• Documentation:
  • Updated README.md to explain the new multi-provider functionality and configuration process.
• Testing:
  • Introduced Jest for unit testing.
  • Added tests for getLLMService, OpenAILLMService, and the stubbed provider classes.

This refactoring allows for easier integration of new LLM providers in the future and gives you more flexibility in choosing your preferred service.