chat: lowercase 'artifacts' in artifacts widget header

[mokashang]

Lowercases the artifacts count label in the chat artifacts widget header so it visually aligns with the adjacent lowercase "file change" label.

Per @roblourens in #309415: "I would probably go lowercase on artifacts".

Change

src/vs/workbench/contrib/chat/browser/widget/chatArtifactsWidget.ts

-    ? localize('chat.artifacts.one', "1 Artifact")
-    : localize('chat.artifacts.count', "{0} Artifacts", data.totalCount);
+    ? localize('chat.artifacts.one', "1 artifact")
+    : localize('chat.artifacts.count', "{0} artifacts", data.totalCount);

The localize keys are unchanged; only the source English strings change.

How to test

1. Open a chat session that produces multiple artifacts (e.g. via an agent that emits files).
2. Confirm the artifacts widget header now reads N artifacts (or 1 artifact) instead of N Artifacts.
3. Confirm the header still aligns visually with the lowercase "file change" badge.

Closes #309415

[Copilot]

Pull request overview

Updates the chat artifacts widget header text in the VS Code workbench chat UI to use lowercase “artifact(s)”, aligning its casing with the neighboring lowercase “file change” label.

Changes:

• Lowercase the singular label from “1 Artifact” to “1 artifact”.
• Lowercase the plural label from “{0} Artifacts” to “{0} artifacts” while keeping localization keys unchanged.