Fix bt eval for ESM-only dependencies

[parkerhendo]

Summary

• When a TypeScript eval imports an ESM-only package ("type": "module" with only "import" exports), tsx v4 fails because it compiles .ts → CJS regardless of load method. The CJS require() can't match the "import" condition in package.json exports, producing ERR_PACKAGE_PATH_NOT_EXPORTED.
• Adds automatic retry: when tsx fails with a known ESM interop error, retries the eval using vite-node with BT_EVAL_FORCE_ESM=1, which skips the require() path in the eval-runner and uses Vite's bundler-based module resolution instead.
• Adds a test fixture (eval-ts-esm-only-dep) with a local ESM-only package to verify the fix.

Test plan

• cargo test eval_fixtures passes with the new eval-ts-esm-only-dep fixture
• Existing eval fixtures continue to pass (tsx path unchanged when no ESM error)
• Manual: bt eval --runner tsx against a project with ESM-only deps retries and succeeds

[github-actions]

Latest downloadable build artifacts for this PR commit ba6623b97f5f:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/22330664157
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