🧹 Code Health: Implement missing getDefaultMemoValue logic

[ManupaKDU]

🎯 What: Replaced the unimplemented TODO logic inside getDefaultMemoValue in LimsMiddlewareController with actual fallback logic.
💡 Why: Having unimplemented logic in production code is generally bad for maintainability and readability, and might cause incorrect program state downstream when missing values are returned silently. This syncs the implementation with the identical signature in PatientReportBean.
✅ Verification: Modified the file with replace_with_git_merge_diff, verified by reading output with sed, and ensured all unrelated code tests passed with mvn clean test.
✨ Result: The LimsMiddlewareController now properly resolves the default memo value based on whether the investigation item exists and has HTML text, preventing empty returns and matching system conventions.

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PR created automatically by Jules for task 7458592376034505133 started by @manupawickramasinghe

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