Fix stale BYOK model selection after vendor resolves with zero models

[Copilot]

After removing BYOK models and reloading, the chat picker button kept displaying the previously selected BYOK model (e.g. "Claude Haiku 4.5") while the dropdown only listed "Auto".

Root cause

The chat input bridges the startup race by retaining cached models for vendors that haven't resolved yet (mergeModelsWithCache), keying eviction on hasResolvedVendor. When a BYOK vendor later resolves with zero model groups (key/config removed), _modelsGroups.set(vendor, []) flips hasResolvedVendor from false → true, but _resolveAllLanguageModels did not fire onDidChangeLanguageModels because both the old and new model/group lists were empty. Consumers silently evicted the cached BYOK model from their merged view, yet resetCurrentLanguageModelIfUnavailable never ran, so _currentLanguageModel kept pointing at the now-unavailable model.

Changes

• src/vs/workbench/contrib/chat/common/languageModels.ts: treat the first resolution of a vendor as a change, so the event always fires on the unresolved → resolved transition. The existing reset handler then falls back to the default model in the list (per the agent instruction on the issue).

  const wasUnresolved = !this._modelsGroups.has(vendorId);
  const oldGroups = this._modelsGroups.get(vendorId) ?? [];
  this._modelsGroups.set(vendorId, languageModelsGroups);
  const oldModels = this._clearModelCache(vendorId);
  let hasChanges = wasUnresolved;

• src/vs/workbench/contrib/chat/test/common/languageModels.test.ts: regression test "fires onChange event when a vendor first resolves with zero models".