fix: remove namespace references from error messages

[turip]

Overview

Fixes:

• https://github.com/openmeterio/openmeter/pull/904#discussion_r1601411573
• OM-531
• CreateLedgerGrantRequest must not have id as a mandatory field

Notes for reviewer

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes