fix(vscode): load mention-utils.js in the chat-panel test harness

[addyCooks]

Description

chat-panel.js destructures globalThis.NanocoderMentionUtils, which media/mention-utils.js
defines. The webview loads both scripts in that order (chat-webview-provider.ts:486,489), but
source/vscode/chat-panel-harness.ts only ran chat-panel.js so createPanel() threw before
any assertion could run:

TypeError: Cannot destructure property 'findMentionQuery' of
'globalThis.NanocoderMentionUtils' as it is undefined.
    at evalmachine.<anonymous>:150:10
    at createPanel (source/vscode/chat-panel-harness.ts:240:2)

That is 28 failures chat-panel-thoughts (13) and chat-panel-tool-cards (15) — on main
and on every branch cut from it. The harness was missed when the mention helpers were split out
into their own script in #842; mention-utils.js even documents the contract in its header
("Loaded as a plain script before chat-panel.js").

Runs mention-utils.js in the same VM context first, matching the load order the webview
already uses. One file, +8 lines.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging

[github-actions]

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