Fix: restore technician labels on stacked bar charts

[Rom1-B]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !46505
• Technician names stopped appearing on the "Number of tickets per status and technician" stacked bar chart, with no error in the logs.
• The labels array was JSON-encoded as a keyed object while the chart rendering code read it by positional index, so the lookup always returned nothing.
• Labels are now re-indexed before being sent to the chart, so technician names display correctly again on all affected stacked/area/line SVG charts.

Screenshots (if appropriate):