test: remove untestable file upload error tests

Remove file-upload-errors.test.mts as the error handling code it targeted
is unreachable. Node.js fs.createReadStream() does not throw synchronously;
it emits errors on the stream later, making the try-catch block in
file-upload.ts:31-46 defensive dead code that cannot be covered.

The error handling paths (ENOENT, EACCES, EISDIR) on lines 34-45 are
intentionally unreachable and do not need test coverage.

Author: jdalton