🧪 Add missing error path test for JSON parsing in fetchLatestDMG

[hatimhtm]

🎯 What: Added test coverage for the error path where JSON parsing fails in fetchLatestDMG. Refactored fetchLatestDMG to accept an injected URLSession for mocking network responses, and implemented a fallback to open the browser if the latest .dmg asset is not found or JSON parsing fails.
📊 Coverage: Covered the network request and JSON parsing error handling in fetchLatestDMG by injecting a MockURLProtocol and returning malformed JSON.
✨ Result: Increased test coverage and confidence in the app's upgrade path.

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PR created automatically by Jules for task 7611284505251264038 started by @hatimhtm

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