fix(api): resolve async user lookup route params

[rissrice2105-agent]

Summary

• Resolve Promise-based Next.js route params in user lookup endpoints before reading username or identifier
• Prevent valid /api/users/by-username/[username] and /api/users/by-id/[identifier] requests from querying with undefined params
• Add regression tests for async route params and missing param handling

Tests

• corepack pnpm exec vitest run --config $env:TEMP/qryptchat-vitest-no-react.config.mjs "src/app/api/users/by-username/[username]/route.test.js" "src/app/api/users/by-id/[identifier]/route.test.js"
• corepack pnpm exec oxlint "src/app/api/users/by-username/[username]/route.js" "src/app/api/users/by-username/[username]/route.test.js" "src/app/api/users/by-id/[identifier]/route.js" "src/app/api/users/by-id/[identifier]/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.