🧪 [Testing] Add missing error path test for JSON parsing in fetchLatestDMG

[hatimhtm]

🎯 What: Added missing tests for the JSON parsing error path in AppDelegate.fetchLatestDMG. Modified AppDelegate to use an injectable URLSession, exposed fetchLatestDMG, and added XCTAsserts to the new tests to actually verify the behavior.

📊 Coverage: Tests now cover:

• Invalid JSON string responses (verifies exactly 1 request made, download not triggered)
• Valid JSON string responses with missing 'assets' array (verifies 1 request made)
• Valid JSON string responses with no '.dmg' suffix in 'assets' array (verifies 1 request made)
• Happy path (valid JSON with '.dmg' suffix, verifies 2 requests made)

✨ Result: Real test assertions implemented. Ensured fetchLatestDMG handles invalid and valid responses gracefully and falls back correctly. Test coverage has improved, acting as a safety net.

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PR created automatically by Jules for task 17390460225234376522 started by @hatimhtm

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