Run duplicate checks during schema-only validation

[be-student]

Summary

• run duplicate source-document checks for every current terms type during schema-only validation
• keep browser and network source-document checks behind full validation
• add a CLI regression fixture and document the patch-level change

Closes #1265

Validation

• npm run test:only -- scripts/declarations/validate/index.test.js scripts/declarations/validate/schemas.test.js (20 passing)
• NODE_ENV=test npm run declarations:validate:schema -- --services service_with_multiple_source_documents_terms (4 passing, including duplicate validation)
• npx eslint scripts/declarations/validate/index.test.js scripts/declarations/validate/index.mocha.js
• git diff --check

The full npm test run reached 1,250 passing tests. Three Mongo setup hooks timed out because no local Mongo service was available, and the existing dataset export test found a generated README mismatch. The new CLI regression initially hit Mocha's two-second suite timeout and now has an explicit ten-second limit; it passes in 0.7-1.1 seconds.