fix(trust-graph): map write identity lookup errors

[EfeDurmaz16]

Summary

• map incident and revocation identity discovery failures to 404/503 instead of generic write 400s
• document 400/404/503 write responses in the trust-graph OpenAPI spec
• add route coverage for missing and unavailable reporter/revoker identities

Verification

• pnpm --filter @fides/trust-graph test
• pnpm --filter @fides/trust-graph lint
• pnpm build
• git diff --check