[routes-b] GET,PATCH /api/routes-b/invoices/[id]/client - get and update invoice client

[davedumto]

Summary

Rebuild the removed invoices/[id]/client endpoint under /api/routes-b. get and update invoice client.

Requirements

• Restore handler at app/api/routes-b/invoices/[id]/client/route.ts
• Implement methods: GET,PATCH
• Add request validation, auth and ownership checks, and structured error handling
• Add unit tests for the happy path and key failure modes

Acceptance criteria

• Handler restored for each listed method at the documented path
• Validation and error envelopes match existing routes conventions
• Unit tests pass in CI