Use terminal-secure icon for sandboxed commands (#303778)

* Adopt new codicons version

* terminal: use terminal-secure codicon for sandboxed commands

Add per-invocation icon support to tool invocations via
`IPreparedToolInvocation.icon` and `IChatToolInvocation.icon`.
The terminal tool sets the icon to `terminal-secure` when sandbox
is active, or `terminal` otherwise. The thinking content part and
subagent content part use this icon when rendering, falling back
to the existing heuristic for tools without a registered icon.

Also removes the $(lock) theme icon prefix from sandbox invocation
messages since the icon now communicates the sandbox state.

* Fix unit test

* Review feedback