Fast-track companion to #37643.
GitHub's `workflow_dispatch` event is only discoverable when the
workflow file exists on the default branch — even though the workflow
code that runs comes from the `ref` passed to the dispatch. This PR
lands the `Middleware Evals` workflow file on master so that #37643
(which adds `libs/langchain_v1/tests/evals/`) can be dispatched against
the feature branch via:
```bash
gh workflow run middleware_evals.yml \
--ref nh/todo-middleware-loop-contract \
--field models='claude-sonnet-4-6,...'
```
without first merging the full eval framework.
## Caveats
- The workflow's pytest invocation depends on
`libs/langchain_v1/tests/evals/` and the partner SDK list, neither of
which exists on master yet. Dispatching with `--ref master` before
#37643 lands will fail at pytest collection. That's the intended
behavior — the workflow's purpose is to dispatch against branches that
ship the eval suite.
- Once #37643 merges to master, this workflow file already matches what
#37643 adds. The merge will be a no-op for `middleware_evals.yml`
itself.
