tests: characterize macOS sandbox link writes

[bolinfest]

Why

#21819 adds high-level coverage for sandboxed apply_patch and sandboxed fs/writeFile behavior around link aliases. This follow-up tests the lower-level macOS workspace-write sandbox directly so the boundary is explicit.

The important distinction is that writing an existing hard link inside the writable workspace preserves normal filesystem semantics, but that is not an arbitrary-write primitive by itself: the sandbox rejects creating a new hard link to a file outside the workspace, and it rejects symlink write-through to an outside target.

What Changed

Added macOS-only codex-exec sandbox tests that verify workspace-write:

• allows writing through an existing hard link inside the workspace, mutating the shared inode;
• rejects writing through an existing symlink to an outside file;
• rejects creating a hard link to an outside file from inside the sandbox.

Testing

• cargo test -p codex-exec macos_workspace_write
• just fix -p codex-exec

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• -> tests: characterize macOS sandbox link writes #21845
• tests: cover sandbox link write behavior #21819