[PM-5997] Fix forum DTO handling and topic deletion policy

[jmgasper]

Summary

• Treat transformed optional DTO properties with undefined values as omitted, fixing top-level comments and title-only topic edits (PM-6169, PM-6135).
• Keep topic editing available to authors while restricting topic deletion to administrators and scoped service callers, without granting deletion through challenge-copilot elevation (PM-6164).
• Add regression coverage using real transformed DTO instances and repair the moderation integration fake so it preserves Prisma promise behavior.

Verification

• 126 tests passed; 3 environment-dependent tests skipped.
• pnpm lint passed.
• pnpm build passed.