test(assets): add unit tests for transfer subcommand

[iborazzi]

Description

Adds unit test coverage for assetsCommand's transfer subcommand in src/commands/assets.ts, which previously had no tests.

Details

• Tests that the command registers the transfer subcommand correctly.
• Tests that a valid --body JSON file is read and POSTed to /api/v2/assets/transfer.
• Tests error handling when the --body file does not exist.
• Tests error handling when the --body file contains invalid JSON.

All 362 tests pass locally (npx vitest run), including the 4 new tests.