[17.0][IMP] hr_course: add reference code to courses and schedules

[miquelrosell99]

This PR adds a reference code field to both `hr.course` and `hr.course.schedule`.

Changes:

• Added `code` (Reference) field to `hr.course` and `hr.course.schedule`
• Auto-generates codes via `ir.sequence` on create if not provided
• Sequences are defined with `noupdate=1` following Odoo core practices:
  • Courses: prefix `C`, padding 5 (e.g., `C00001`)
  • Schedules: prefix `CS`, padding 5 (e.g., `CS00001`)
• Added unique SQL constraints on `code` for both models
• Updated views (form, tree, search) to display the reference
• Added `name_search` support and `display_name` formatting (`[CODE] Name`)
• Added tests for auto-generation, uniqueness, and search by code

This aligns with standard Odoo patterns used in `sale.order`, `purchase.order`, etc.