Expose large shell output artifact paths through the Copilot SDK

Part of: https://github.com/microsoft/vscode/issues/328596
Follow-up from: https://github.com/microsoft/vscode/issues/329720

When the Copilot runtime spills large shell output to a file, the tool completion text includes the saved-file path, while structured `shell_exit` content exposes only `outputPreview` and `outputTruncated`.

VS Code therefore has no typed way to surface or act on the full-output artifact. The immediate UI fix uses the tool completion display text when the structured preview is truncated, but deliberately does not parse a path out of that prose.

The public SDK/runtime contract should expose the large-output artifact location as structured shell completion data so consumers can render it without depending on presentation text.

Acceptance criteria:

- Add an optional structured artifact path or URI for spilled shell output to the runtime wire contract and generated public SDK types.
- Define enough lifetime/access semantics for an SDK consumer to know when the artifact can be opened.
- Update VS Code Agent Host to consume the structured value instead of relying on the tool completion display text.