fix(discord): render provider model lists longer than 25 options across multiple select menus

[EpicIsTheOne]

Summary

The Discord /model picker built a single discord.ui.Select and filled it with models[:25], silently dropping any models beyond the first 25. Discord caps a single select at 25 options, but a message may contain up to 5 component rows — so the fix partitions the model list across up to 3 select menus (25 each; the other 2 rows are Back/Cancel), instead of truncating.

Why

Providers like Nous expose more than 25 models (curated list + Portal recommendations). On Discord the tail was clipped, so e.g. free-tier :free Portal picks (tencent/hy3:free, stepfun/step-3.7-flash:free, etc.) never appeared in the picker — they surfaced fine in the Portal UI / CLI, which don't use Discord's select widget. This makes the Discord picker match the other surfaces.

Changes

• ModelPickerView._build_model_select: slice the model list into ≤25-option chunks and emit one discord.ui.Select per chunk (custom_id model_model_select_<i>), all routed through the existing _on_model_selected handler. Multi-row menus get a (n/total) suffix in the placeholder.
• _on_provider_selected: the "N more available" count now reflects models actually rendered across the partitioned menus rather than the old min(len, 25).
• Added regression test test_discord_model_picker_partition.py asserting all 37 Nous models render (including the :free tail), no truncation/dupes, and the per-menu 25-option cap holds.

Notes

• Caps at 3 menus (75 models) given the 5-row limit; providers exceeding that would still clip, but none currently do.
• No behavior change for providers with ≤25 models (still a single select).

Test plan

• pytest tests/gateway/test_discord_model_picker.py tests/gateway/test_discord_model_picker_partition.py — 6 passed.
• Verified live: list_authenticated_providers(current_provider="nous") emits 37 models; the Discord view now renders all 37 across 2 selects (25 + 12), with the 5 :free picks visible.