Chat window model picker enhancements for multiple providers

[vritant24]

Groups models in the "Other Models" section by providers using titled separator headers.

Changes

• Provider separator headers in "Other Models" (hidden when all models share one vendor)
• Inline provider label next to model name in the promoted/recently-used section (only when multiple vendors exist)
• Provider removed from right-side detail column to avoid duplication
• Search/filter preserves vendor separator headers above matching results
• Display names sourced from the language model provider API (e.g. "Anthropic", "Google") rather than raw metadata

[Copilot]

Pull request overview

Enhances the chat model picker UX to better support multiple language model providers by grouping “Other Models” by vendor, optionally showing vendor labels inline for promoted items, and preserving provider grouping context during search/filtering.

Changes:

• Add vendor display-name resolution and use it to group “Other Models” with titled vendor separators.
• Show an inline vendor label for promoted items when multiple vendors exist, and suppress duplicate vendor info in the detail/description.
• Update the generic ActionListWidget filtering behavior to keep labeled separators visible above matching results; add CSS + tests.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/widget/input/chatModelPicker.ts	Adds vendor display-name lookup, vendor-grouped “Other Models” separators, and inline vendor labels for promoted items.
src/vs/platform/actionWidget/browser/actionList.ts	Adjusts filtering to preserve labeled separators above matching results; updates separator row height handling.
src/vs/platform/actionWidget/browser/actionWidget.css	Styles titled separators (separators with non-empty labels).
src/vs/workbench/contrib/chat/browser/widget/media/chat.css	Tweaks flex behavior so inline vendor labels fit beside promoted model titles.
src/vs/workbench/contrib/chat/test/browser/widget/input/chatModelPicker.test.ts	Updates/extends tests for vendor grouping, header omission for single vendor, and inline vendor label behavior.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/test/browser/widget/input/chatModelPicker.test.ts:468

• The comment says vendors are "sorted alphabetically", but the implementation sorts copilot first and then alphabetically by display name for the rest. Updating the comment to match the actual ordering will prevent confusion (and makes it clearer why 'Copilot' is expected first here).

		// Vendors sorted alphabetically by display name
		assert.strictEqual(vendorSeparators[0].label, 'Copilot');
		assert.strictEqual(vendorSeparators[1].label, 'Other-vendor');
		// Models within each vendor group are sorted alphabetically

• Files reviewed: 5/5 changed files
• Comments generated: 5