🧪 [test] add unit tests for core logic function

[gasatrya]

This PR addresses the missing test file for the core logic in src/core/logic.ts.

🎯 What: Implemented unit tests for the getEffectiveProviderId function.
📊 Coverage:

• useLocal: false scenario (returns default provider).
• useLocal: true scenario (returns local provider).
• Trimming of localProvider name.
• Fallback to 'ollama' when localProvider is empty or whitespace.
  ✨ Result: Improved test coverage for core configuration logic, ensuring reliable provider selection for users.

Note: Standard npm test was not run due to network timeouts in the environment preventing npm install and tsc from running, but the logic was manually verified using a custom Node.js script.

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PR created automatically by Jules for task 3532180536030445168 started by @gasatrya

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