Web Shell mention icon chips

Problem

The custom @ mention menu can insert extension, file, and MCP references, but accepted items were rendered as plain text in the composer. A previous composer path rendered these references as icon chips. The current custom mention architecture also needs a way for host-defined mention items, such as tables, to use the same chip rendering.

Design

• Keep the @ mention menu responsible for choosing and inserting text.
• Let mention items optionally provide a composerTag that describes the inserted reference.
• Continue to auto-create composer tags for built-in file, extension, and MCP providers so existing built-in mentions regain icon chips without host changes.
• Add a composerTagIcons prop on WebShell so hosts can register icon URLs by composerTag.kind.
• Resolve icons at composer rendering time through one helper that checks custom icons first and falls back to built-in icons.
• Store resolved icon URLs only in the internal inline decoration data and strip them from public composer tag values.

Scope

This change covers composer tag icon registration and rendering for accepted @ mention items and programmatically inserted inline tags. It does not change the visible @ mention picker rows or add a new provider registration API beyond the existing atProviders surface.

Risks

• Custom icon URLs are applied through CSS masks, so URL values must be escaped before writing CSS custom properties.
• Existing inline decorations need to refresh if composerTagIcons changes while text remains in the editor.