fix(inspect): scope middle-out transform to OpenRouter

[Danconnolly]

Summary

• Remove the unconditional middle-out transform from the controlled sandbox solver.
• Resolve the configured model once per generation.
• Apply transforms=["middle-out"] only when the model is routed through OpenRouter, matching the existing unbounded-solver behavior (see lines 591-602).

Why

middle-out is an OpenRouter-specific input-context transform. Sending it to direct or OpenAI-compatible non-OpenRouter model providers can cause unsupported-parameter failures.

Testing

• uv run python -m compileall -q fle/eval/inspect/sandbox/sandbox_solver.py
• git diff --check