fix: validate JWT exp claim in Go services using ParseWithClaims

[Senthil455]

Changes

Fixed JWT expiration validation across three Go services where jwt.Parse with MapClaims silently discarded type-assertion errors and did not automatically validate the exp claim, allowing tokens to remain valid forever.

Files changed:

• services/attendance-service/auth.go — Replaced jwt.Parse with jwt.ParseWithClaims using a typed internalClaims struct embedding jwt.RegisteredClaims. Removed silent _ type-assertion discards.
• services/lms-service/middleware/auth.go — Same fix as attendance-service.
• services/notification-go-service/main.go — Same fix for both validateJWT and internalAuthMiddleware functions.

What changed:

• Added custom claims structs embedding jwt.RegisteredClaims (enables automatic exp, nbf, iat validation)
• Switched from jwt.Parse to jwt.ParseWithClaims for typed claim parsing
• Replaced claims["key"].(string) silent discards with typed field access
• Removed redundant !token.Valid checks (handled by ParseWithClaims with RegisteredClaims)