fix: remove backslash escaping from terminal command labels (#303856)

* fix: remove escapeMarkdownSyntaxTokens from terminal command labels

Fixes #303844

The command text in ChatTerminalThinkingCollapsibleWrapper was being
escaped with escapeMarkdownSyntaxTokens(), which adds backslashes before
chars like - * # etc. This is unnecessary because the text is always
rendered inside markdown code spans or via .textContent, both of which
treat content as literal.

Also adds a component fixture for the terminal collapsible wrapper to
enable visual regression testing of command label rendering.

* fix: use DOM nodes instead of MarkdownString for sandbox command labels

Addresses review feedback: commands containing backticks (common in
PowerShell) would break the inline-code markdown spans. Now both
sandbox and non-sandbox paths use text nodes + <code> elements with
.textContent, which is always safe for arbitrary command text.

Also adds fixture cases for backtick-containing commands to catch
this class of issue.

* fix: remove colons from fixture names to fix CI artifact paths

* add screenshot baselines for terminal collapsible fixtures

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