fix(files): reject blank message file ids

[aiirvizionz]

Summary

• Normalize messageId in the message-files API route before validation and database filtering.
• Reject blank/whitespace-only message ids before doing auth or Supabase work.
• Add regression coverage for whitespace-only route params.

Validation

• git diff --check

I attempted pnpm vitest run src/app/api/files/message/[messageId]/route.test.js, but this Windows checkout stops during dependency preparation because the package postinstall script uses POSIX shell commands (command -v ... || true). I also attempted direct Vitest execution with node node_modules\vitest\vitest.mjs run "src/app/api/files/message/[messageId]/route.test.js", but the local Vitest config fails to load because @vitejs/plugin-react imports vite/internal, which is not exported by the installed Vite package. I did not modify dependency files.