fix(news-aggregator): parse limits strictly

[rissrice2105-agent]

Summary

• add a shared strict positive-integer limit parser for news-aggregator
• use it in both the HTTP search endpoint and MCP tool endpoint
• add regression coverage so malformed values like 2abc no longer get treated as 2

Tests

• corepack pnpm install --frozen-lockfile
• corepack pnpm install (inside mcp_modules/news-aggregator)
• corepack pnpm exec mocha --require=./test/setup.js mcp_modules/news-aggregator/test/controller.test.js
• git diff --check

Note: root eslint on the touched news-aggregator files still reports pre-existing indentation style errors across the module, so I did not reformat the module in this focused bug fix.