[18.0][MIG] fieldservice_isp_flow: Migration to 18.0

[fsmw]

Migration of fieldservice_isp_flow module to Odoo 18.0.

Summary

• Updated module version to 18.0.1.0.0
• No framework changes required (module uses compatible patterns)
• Adds ISP-specific workflow stages and validation logic

Module Description

This module provides a specialized workflow for Internet Service Providers, adding stages like Confirmed, Requested, Assigned, Scheduled, En Route, and Started with appropriate validation rules for each transition.

Closes part of migration checklist in OCA/field-service#1220