fix: restore file-command and VSIX input boundaries

[PathGao]

Summary

• Restore the theme and image command path boundaries from fix: constrain theme and image file paths #249 after Revert "fix: constrain theme and image file paths" #269 reverted it solely to correct the merge order with fix: prevent UTF-8 fence scanner panics #265.
• Bound VSIX HTTP downloads, archive expansion, and JSON reads.
• Keep the fix: prevent UTF-8 fence scanner panics #265 UTF-8 fenced-code regression test alongside the restored path tests.

Context

#249 was merged before #265 and then reverted by #269. @alecdotdev confirmed that the implementation itself had no issue and requested a new PR after #265.

Related: #248, #250, #249, #269.

Validation

• npm run check
• npm test (117 passing)
• cargo test (20 passing)

[PathGao]

@alecdotdev Reopened as requested on top of #265. This restores #249 unchanged except for retaining #265's UTF-8 fence regression test; all local checks are passing.