chore: Keep view_image sandbox test in temp dir

[jif-oai]

Summary

• move the view_image sandbox filesystem-read unit test onto a temporary cwd
• keep the turn cwd and selected turn environment cwd aligned inside the test
• avoid leaving core/image.png behind in the repo checkout after the test runs

Root cause

The test wrote image.png beneath turn.cwd, and the shared session test helper defaults that cwd to the current repo directory when no override is provided.

Validation

• just fmt
• cargo test -p codex-core tools::handlers::view_image::tests::handle_passes_sandbox_context_for_local_filesystem_reads