fix: guard against KeyError in telechat_page_count

[jennifer-richards]

Fixes server errors when a pre-AD views the agenda_documents view.

The root cause of the issue is including pre-AD roles here:

datatracker/ietf/iesg/views.py

Line 363 in cb25831

ad = request.user.person if has_role(request.user, "Area Director") else None

The ad value set in that view is only used to compute the pages left to ballot, where it's assumed that the ad will be an active balloter. The bug being fixed comes from that assumption being violated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (896968d) to head (9fa6176).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8485   +/-   ##
=======================================
  Coverage   88.80%   88.80%           
=======================================
  Files         312      312           
  Lines       40886    40856   -30     
=======================================
- Hits        36307    36284   -23     
+ Misses       4579     4572    -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.