Allow original imported files on glTF export commands

[Joe-Spencer]

Allow export and export3d to carry an optional imported_files array of original project-relative paths and raw bytes. glTF exports use it to retain imported CAD inputs and dependent files alongside KCL source; other output formats ignore it. Reuse ImportFile so binary contents retain their existing wire encoding and redacted debug behavior.

Stacked on #1393. Older requests and builders default to an empty array, and empty arrays are omitted from serialized commands. JSON/BSON round-trip tests cover both command variants with non-UTF-8 and empty files; the full websocket OpenAPI schema is regenerated with KCL execution retained.

Companion drafts: format #925, engine #5303, and Studio #14442.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 36.64%. Comparing base (305097d) to head (cee7bb0).

Additional details and impacted files

@@                     Coverage Diff                     @@
##           codex/export-kcl-source    #1442      +/-   ##
===========================================================
+ Coverage                    36.12%   36.64%   +0.51%     
===========================================================
  Files                           32       32              
  Lines                         1733     1733              
===========================================================
+ Hits                           626      635       +9     
+ Misses                        1107     1098       -9     

Flag	Coverage Δ
unittests	36.64% <ø> (+0.51%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.