🧪 [testing improvement] Add comprehensive tests for SnippetManager.ImportSnippetsAsync

[Avicennasis]

🎯 What: The testing gap addressed
The SnippetManager.ImportSnippetsAsync method lacked comprehensive testing, particularly around verifying that imported snippets are appended to the collection rather than overwriting it, and that the resulting combination is correctly persisted to disk based on the session's encryption state.

📊 Coverage: What scenarios are now tested

• Successfully appending to an existing collection using a plain JSON file and verifying it saves to disk as plain text.
• Successfully appending to an existing collection using an encrypted JSON file with an explicit PIN, and verifying it saves to disk.
• Successfully appending to an existing collection using an encrypted JSON file relying on the session PIN, and verifying the background save correctly encrypts the output on disk.
• Properly returning false and logging an error if an exception (like IOException) is thrown while attempting to read the import file.

✨ Result: The improvement in test coverage
SnippetManager.ImportSnippetsAsync is now robustly tested against its core functionality and known error paths, ensuring regressions in append behavior or encryption persistence are immediately caught by the unit tests.

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PR created automatically by Jules for task 14116381366135983054 started by @Avicennasis

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[Avicennasis]

Conflicts with earlier test PRs (#93, #98) that already modified SnippetManagerTests.cs. Import test coverage can be added in a follow-up.