fix(gym): isolate scenario Cargo projects from parent workspace

[kyledef]

Summary

Two open-model-gym scenarios (file-editing and multi-turn-edit) scaffold a Cargo project without declaring [workspace]. When the gym is run from inside a checkout of goose itself — which is the most natural place to run it — the final cargo build validation step fails before goose's output is even evaluated:

error: current package believes it's in a workspace when it's not:
current:   .../evals/open-model-gym/suite/.workdir/file-editing_.../Cargo.toml
workspace: .../goose/Cargo.toml

This causes the scenario to be scored as failed regardless of how well the agent performed the edit.

Fix

Add an empty [workspace] table to the scaffolded Cargo.toml in each affected scenario. This makes the scenario project self-contained so cargo build works regardless of where the gym is executed.

  Cargo.toml: |
    [package]
    name = "user-service"
    version = "0.1.0"
    edition = "2021"
+
+   [workspace]

Repro

From inside a goose checkout, with a model configured:

cd evals/open-model-gym
just install
just run  # or: npx tsx suite/src/runner.ts

Before this change, file-editing fails at the cargo build validation rule even when the agent produces the correct edit. After the change, the scenario runs to completion.

Verified locally on macOS with databricks/goose-claude-4-7-opus — the file-editing scenario now passes end-to-end (1/1 tests passed in ~27s).

Scope

Minimal, two-line change. No behavior change for users who run the gym outside a Rust workspace.