Replace magic number with DhwOperationSetting.VACATION enum

[Copilot]

Addresses regression in PR #47 where vacation mode validation used magic number 5 instead of the DhwOperationSetting.VACATION enum introduced in that PR.

Changes

• Import: Added DhwOperationSetting enum to mqtt_device_control.py
• Condition check: Replaced if mode_id == 5: with if mode_id == DhwOperationSetting.VACATION:
• Error messages: Updated validation messages to reference enum value instead of hardcoded 5
• Docstring: Updated parameter documentation to reference the enum

# Before
if mode_id == 5:  # Vacation mode
    if vacation_days is None:
        raise ParameterValidationError(
            "vacation_days is only valid for vacation mode (mode 5)",
            parameter="vacation_days",
        )

# After
if mode_id == DhwOperationSetting.VACATION:
    if vacation_days is None:
        raise ParameterValidationError(
            f"vacation_days is only valid for vacation mode "
            f"(mode {DhwOperationSetting.VACATION})",
            parameter="vacation_days",
        )

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.