PM-4904: Allow project billing account clears

[jmgasper]

What was broken
Clearing a billing account from an existing project did not persist. The Work UI sent an explicit null billingAccountId, but the Projects API treated that value as omitted, so the old billing account stayed on the project and appeared again after save.

Root cause
UpdateProjectDto inherited CreateProjectDto billingAccountId parsing, which normalizes null and empty string to undefined. The service clear path depended on a derived flag that is not materialized by class-transformer when only billingAccountId is sent.

What was changed
UpdateProjectDto now owns the update-time billingAccountId field, preserving null and empty string as explicit clear requests while still parsing numeric billing account ids. ProjectService now clears when the DTO billingAccountId is null, in addition to the existing internal clear flag.

Any added/updated tests
Added DTO coverage for null, empty string, and numeric billingAccountId updates. Updated the project service clear test to exercise billingAccountId: null directly.

Validation

• pnpm test -- --runTestsByPath src/api/project/dto/update-project.dto.spec.ts src/api/project/project.service.spec.ts: passed
• pnpm lint: passed with existing AJV strict-mode warnings
• pnpm build: passed
• pnpm test: failed in unrelated metadata specs that still expect publishProjectEvent calls even though publishMetadataEvent is documented as a no-op