Harden CLI demos against invalid input and validate HashTable size

[Nullkernel]

Motivation

• CLI demo programs used new Scanner/nextInt in many places and would crash or exit abruptly on non-integer input.
• The HashTable initializer allocated the table without validating size > 0, which could cause runtime errors.
• Provide a small, consistent input-recovery UX so demos are more reliable for real users.

Description

• Changed HashTable.initialise() to initialise(Scanner) and added readInt/readPositiveInt helpers plus a INVALID_INT_MESSAGE constant to validate size > 0 before allocating the table in Data-Structures/HashTable.java.
• Replaced ad-hoc nextInt() calls with guarded reads in Data-Structures/Stack.java by adding readInt/readPositiveInt helpers and using them for menu choices and size/value inputs.
• Added the same guarded input helpers and positive-size validation to Data-Structures/Queue/Queue.java and Data-Structures/Queue/CircularQueue.java, and standardized invalid-choice messages across the updated demos.
• Ensured Scanner instances are used consistently (single Scanner passed into initialise for HashTable) and closed at program exit where appropriate.

Testing

• Compiled the modified demos with javac Data-Structures/HashTable.java and javac Data-Structures/Stack.java and javac Data-Structures/Queue/Queue.java and javac Data-Structures/Queue/CircularQueue.java, and compilation succeeded for all four files.

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Codex Task