fix(files): resolve async file route params

[rissrice2105-agent]

Summary

• Resolve Promise-based Next.js route params before reading file and message ids in file APIs
• Return explicit 400 responses for missing messageId/fileId instead of querying with undefined ids
• Add regression coverage for /api/files/message/[messageId] and /api/files/[fileId]/encrypted
• Remove an unused helper from the message files route

Tests

• corepack pnpm exec vitest run --config $env:TEMP/qryptchat-vitest-no-react.config.mjs "src/app/api/files/message/[messageId]/route.test.js" "src/app/api/files/[fileId]/encrypted/route.test.js"
• corepack pnpm exec oxlint "src/app/api/files/message/[messageId]/route.js" "src/app/api/files/message/[messageId]/route.test.js" "src/app/api/files/[fileId]/encrypted/route.js" "src/app/api/files/[fileId]/encrypted/route.test.js"

Note: the default vitest.config.js still fails to load locally because @vitejs/plugin-react imports a non-exported Vite internal path, so the focused route tests were run with a minimal no-React Vitest config.