feat: Enhanced API key validation with better error handling

[utk7arsh]

Summary of Changes

This PR implements comprehensive API key validation for OpenAI, Gemini, and Anthropic providers. The changes include:

1. Enhanced Validation Logic:

   • Added immediate validation on API key entry
   • Implemented provider-specific validation endpoints
   • Added detailed error handling for common scenarios (401, 429, etc.)

2. UI Improvements:

   • Added visual feedback indicators in SettingsView
   • Implemented success messages showing available models
   • Added provider-specific error guidance
   • Added 5-second timeout for status messages

Related Issue

• Closes Validate API Key on Entry to Prevent Invalid Usage #106

Contributor's Self-Review Checklist

Please check the boxes that apply. This is a reminder of what we look for in a good pull request.

• I have read the CONTRIBUTING.md document.
• My code follows the project's coding style and architectural patterns as described in DESIGN_PATTERNS.md.
• I have added or updated relevant tests for my changes.
• I have updated the documentation to reflect my changes (if applicable).
• My changes have been tested locally and are working as expected.

Additional Context (Optional)

• Each provider (OpenAI, Gemini, Anthropic) now validates API keys using their respective validation endpoints
• Error messages are provider-specific and include guidance for common issues:
  • Invalid API key format (including Empty)
  • Rate limiting (429)
  • Authentication failures (401)
  • Network connectivity issues
  • Region/organization access issues.

One of the screenshots

Thanks @Surya-sourav and @samtiz for helping with my first PR in here.

[Surya-sourav]

@utk7arsh A Clean UI Implementation. A noice feedback to the user in the UI , for its API Validation , instead of looking into the elongated terminal Logs :

@samtiz @entry-sanio @jhyang0