fix: normalize chat message pagination

[rissrice2105-agent]

Summary

• add shared normalization for GET /api/chat/messages pagination query params
• fall back for malformed limit/offset values instead of passing NaN or negative values into Supabase range
• cap oversized limits at 100 and cover defaults, malformed input, and caps with regression tests

Fixes #113.

Validation

• node --input-type=module -e "...normalizeMessagePagination assertions..."
• corepack pnpm exec vitest run src/app/api/chat/messages/pagination.test.js (blocked: local Vitest config fails before tests with @vitejs/plugin-react importing vite/internal)
• corepack pnpm exec prettier --check src/app/api/chat/messages/route.js src/app/api/chat/messages/pagination.js src/app/api/chat/messages/pagination.test.js (blocked: missing prettier-plugin-svelte in local install)
• git diff --check