feat(vscode): add search to model selector popover

[zeug-zz]

Adds a case-insensitive search input to the ModelSelector popover so users with many providers/models can quickly filter by text. Search matches against provider name/id and model name/id; a matching provider shows all of its models.

The search input is anchored at the bottom of the upward-opening popover so it stays adjacent to the model button regardless of viewport. While searching, the connected-only filter applies, the footer is hidden, and collapse state is ignored so matching providers are always expanded. An empty state is shown when no matches exist.

Changes

• New i18n keys model.searchPlaceholder and model.noSearchResults across all 8 locales.
• Search input styled to align with the existing Show all providers LinkButton:
  • Font size 11px matches the footer LinkButton.
  • Padding tuned (1px 6px) so the input box height matches the LinkButton's ~19px.
  • .searchBox wrapper uses symmetric 8px padding so the input is vertically centered.
• Placeholder uses --vscode-input-placeholderForeground with a light-grey fallback.

Tests

06-model-selection.test.tsx adds three scenarios:

• Filtering by query narrows the displayed models.
• Disconnected providers are excluded from search results.
• Empty state is shown when no models match.

The filter assertion is scoped to the panel via within(...) since the trigger button always shows the selected model name.