🧪 Add tests for dispatchFetch in live sources

[Matuyuhi]

🎯 What:
Added a missing test file src/background/live/sources/index.test.ts to cover the dispatchFetch function's behavior, particularly its error path when an unknown or invalid LiveSource type is provided.

📊 Coverage:

• Added test verifying that dispatchFetch throws correctly formatted error when no adapter is found for a given LiveSource type.
• Added tests to cover happy paths (delegating to githubAdapter or rssAdapter appropriately based on the LiveSource type).

✨ Result:
The dispatchFetch function in src/background/live/sources/index.ts is now fully unit tested. The new suite passes and ensures no regressions when handling various source types.

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PR created automatically by Jules for task 3319829850808450412 started by @Matuyuhi

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