fix(super-editor): rethrow export docx errors

[xy200303]

Summary

• Re-throw normalized Editor.exportDocx() errors after preserving the existing exception event and console logging.
• Remove undefined from the internal implementation return union.
• Add a regression test that verifies failed converter.exportToDocx calls reject to the caller.

Verification

• pnpm test:editor -- src/editors/v1/tests/export/exportDocx.errors.test.js
• pnpm test:editor -- src/editors/v1/tests/export/exportDocx.errors.test.js src/editors/v1/tests/export/exportDocx.commentsFallback.test.js
• pnpm --prefix packages/super-editor run types:check (fails: existing unresolved @superdoc/font-system declarations and broad pre-existing JS/TS errors)

Closes #3579

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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