fix(messages): reject blank load conversation ids

[aiirvizionz]

Summary

• Trim message-load conversationId values before validation.
• Reject whitespace-only ids before user lookup, participant lookup, message query, or SSE room join.
• Add regression coverage for blank ids, invalid limits, and trimmed valid ids.

Validation

• git diff --check
• node node_modules\vitest\vitest.mjs run --config %TEMP%\qryptchat-vitest-minimal.config.mjs "src/app/api/messages/load/route.test.js" (3 tests passed)

I used the same temporary minimal Vitest config needed locally because the repo's default Vitest config currently fails to load through the installed Vite/plugin combination; no dependency files were modified.