Add live coverage for VS Code chat dispatch

[VeryComplexAndLongName]

Why

The harness-stage-dispatch change's remaining human task needed a live check that the Chat window opens with the built prompt. Its event contract was previously unreachable from the Extension Development Host suite because no test could deliver a real webview command into the panel's message handler.

What changed

• expose a test-only-in-intent command delivery and event observation surface through ExtensionTestApi
• add real Extension Development Host coverage for vscode-chat dispatch and the ordinary agent fallback
• assert started -> handedOff, no completed, and the exact workbench.action.chat.open prompt
• narrow the remaining human check in harness-stage-dispatch to observing the actual Chat window

Validation

• npm run typecheck --workspace openspec-ui-vscode
• npm run lint --workspace openspec-ui-vscode
• npm run test:integration --workspace openspec-ui-vscode (12 passing)
• openspec-ui-cli validate --format text
• git diff --check

No changeset: test-only coverage.