Update sessionFs types for structured error contract and add workspace tests

- Regenerate rpc.ts from updated api.schema.json with structured error type
  (error?: { code: 'ENOENT' | 'UNKNOWN', message?: string }) on all sessionFs results
- Update createTestSessionFsHandler to catch fs errors and return error objects
  instead of throwing (matches the new RPC contract)
- Add E2E tests for workspace metadata and plan.md written via sessionFs
- Add test snapshots for new workspace tests

Depends on: github/copilot-agent-runtime#6479

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS