🛡️ Sentinel: [HIGH] Fix JPQL injection in auto-complete methods

[ManupaKDU]

🚨 Severity: HIGH
💡 Vulnerability: JPQL injection vulnerability in GradeController and StaffLeaveEntitleController auto-complete methods due to string concatenation of user input (qry).
🎯 Impact: Malicious users could potentially inject arbitrary JPQL into the query, bypassing security controls, altering query logic, or extracting sensitive information from the database.
🔧 Fix: Refactored the completeGrade and completeStaffLeaveEntitle methods to use secure, parameterized queries with named parameters (:q) and a query parameter map.
✅ Verification: Ran ./detect-maven.sh test which passed 243 tests successfully. Ensured no regressions were introduced.

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PR created automatically by Jules for task 2398628048851096371 started by @manupawickramasinghe

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