fix(completions): init emoji once in CAPF setup

[liaowang11]

Summary

The re-search-backward in the emoji CAPF functions uses telega-emoji-candidate-max-length as the search bound:

(re-search-backward "\\(?:^\\|[[:space:]]\\)\\(:[^: _]+\\)"
  (max (point-min) (- end telega-emoji-candidate-max-length)) t)

When telega-emoji-candidate-max-length is 0 (before init), the bound collapses to end, leaving no range to search backward — so the regex never matches, the when-let* falls through, and telega-emoji-init (previously inside the match block) is never reached.

Move the init call to telega-completions-setup-capf, where it runs once at chat buffer setup time, only when an emoji CAPF function is present in telega-completions-capf-functions.

Test plan

• All 20 existing tests pass including new emoji CAPF tests
• CI passes on both Emacs 28.1 and snapshot
• Verify emoji completion works in a chat buffer with CAPF
• Verify emoji completion still works with company backend

[zevlg]

Merged in, thank you!