fix: avoid duplicate restoration of explicit directory markers

[jacksonriding]

When LibreChat attaches a directory marker under a renamed display filename, the normalized object listing can return the same stored .dirkeep again under its original name. The sandbox then downloads both references to the stored path and rejects execution.

Suppress the extra listing entry by object ID within the listed session. Explicit downloads still preserve marker contents, and an identical object ID in another session does not suppress an inherited marker.

Validation on Bun 1.4.2:

• All 36 egress gateway tests pass.
• The new regression fails without the listing fix.
• Service build completes, with three TypeScript warnings outside the modified listing logic.