[FIX] bantracking: display assigned technician correctly on report

Updated the Service Order report so the technician name and signature
always correspond to the technician assigned to the task, instead of the
user printing the document.

Previously, the report used `env.user.partner_id`, causing the report to
wrongly display the name of the user generating the printout. The
template now resolves the technician using `doc.user_ids[0].partner_id`,
and includes a safety check to avoid errors when no user is assigned to
the task.

Additionally:
- Corrected the label “Technical” to “Technician” and updated its
translation accordingly.
- Renamed template variables for clarity and consistency.

These fixes ensure the printed Service Order accurately reflects the
assigned technician, meeting the requirement that official
documentation shows the correct responsible person, regardless of who
prints the report.

Author: IrvingReyes