fix(plugins): handle malformed execute JSON

[rissrice2105-agent]

Summary

• return a 400 Invalid JSON body response when POST /api/plugins/execute receives malformed JSON
• avoid initializing or running plugins when the request body cannot be parsed
• add focused regression coverage for the malformed-body path

Tests

• corepack pnpm exec vitest run src/app/api/plugins/execute/route.test.js --config .codex-vitest-no-react.config.mjs (temporary no-React config; default vitest config is blocked locally by @vitejs/plugin-react importing vite/internal)
• corepack pnpm exec oxlint src/app/api/plugins/execute/route.js src/app/api/plugins/execute/route.test.js (passes with pre-existing unused-parameter warnings in route.js)
• git diff --check