feat(tests): add Integration Tests for Dice Commands(EXISTS, EXPIRE, EXPIREAT, EXPIRETIME) and Assertions Package

[RishabhC-137]

This pull request resolves several issues related to Dice command testing (Issue #29):
Added integration tests for Dice commands (EXISTS, EXPIRE, EXPIREAT, EXPIRETIME) in the following test files:

• internal/tests/integration/commands/exists_test.go
• internal/tests/integration/commands/expire_test.go
• internal/tests/integration/commands/expireat_test.go
• internal/tests/integration/commands/expiretime_test.go

Created a reusable assertions package to facilitate result validation across tests:

• Includes the AssertResult function which verifies command execution outcomes using testify/assert.
• Handles both expected error and non-error scenarios by asserting error presence and matching response/error messages.

[RishabhC-137]

@lucifercr07 can you review this