[codex] Title-case drug interaction responses

[Dreamstore2046]

Summary

• normalize drug interaction response names to Title Case while keeping lowercase matching internally
• return the response medications array in Title Case for matched and no-interaction results
• return interaction drug1 and drug2 fields in Title Case regardless of input casing
• add Vitest coverage for all-lowercase, ALL-UPPERCASE, and MixedCase inputs producing identical output

Closes #14

Validation

• npm test -- services/drug-interaction-api/__tests__/check-interactions.test.ts services/drug-interaction-api/__tests__/drug-normalization.test.ts services/drug-interaction-api/__tests__/sort-pairs.test.ts
• npx tsc --noEmit --target ES2022 --module ESNext --moduleResolution bundler --allowImportingTsExtensions --esModuleInterop --allowSyntheticDefaultImports --strict --skipLibCheck --types node,vitest services/drug-interaction-api/logic.ts services/drug-interaction-api/__tests__/check-interactions.test.ts
• git diff --cached --check

Notes

• Both the unified server and the standalone drug interaction service call services/drug-interaction-api/logic.ts, so this keeps their behavior in sync.

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