fix(discord): paginate model picker beyond 25 providers/models

[dragocz1995]

Problem

Discord select menus are capped at 25 options. ModelPickerView truncated both
the provider dropdown (options[:25]) and the per-provider model dropdown
(models[:25]), silently hiding everything past the 25th entry. The gateway
builds the picker with list_picker_providers(max_models=50), so up to half the
models per provider were unreachable from the dropdown — and any deployment with
more than 25 providers couldn't reach the rest at all.

Change

• Split both the provider and model dropdowns into pages of 25 with ◀ Prev /
  Next ▶ buttons.
• Navigation buttons appear only when there is more than one page, and are
  disabled at the first/last page.
• The select placeholder shows a (page X/Y) indicator when paginated.
• Single-page lists render exactly as before (no extra buttons).
• Model pagination resets when switching providers or going Back.

Stays within Discord's view limits (one select row + one button row). The
existing "N more available — type /model <name> directly" hint now only
fires for models beyond the max_models cap rather than the 25-option UI limit.

Testing

Constructed ModelPickerView with 60 providers × 50 models and verified each
page renders ≤25 options, Prev/Next enable/disable correctly at the bounds, and
single-item lists show no navigation buttons.

[teknium1]

Fixed on main via PR #48611 — the interactive model pickers no longer cap at 50 models; large provider catalogs are fully searchable.

This was one of several PRs targeting the same picker cap. We merged @islam666's #48297 as the implementation (cleanest scope — it correctly keeps the Telegram/Discord inline-keyboard cap, which genuinely needs a bound) plus a follow-up tightening max_models=0 semantics and a regression test. Closing this as a duplicate — thanks for the contribution and for flagging the issue.

#48611