perf: cache compiled RegExp for proxy context matching

[tomohiro86]

Summary

• server.proxy contexts starting with ^ were compiled via new RegExp(context) on every incoming request
• Pre-compile them once during proxyMiddleware initialization and reuse via a Map cache

Details

doesProxyContextMatchUrl was called on every request for every proxy context. When a context started with ^, it created a new RegExp object each time — regex compilation is relatively expensive and the pattern never changes after config is loaded.

// before: compiled on every request
(context[0] === '^' && new RegExp(context).test(url))

// after: compiled once at startup, reused from cache
regexCache.get(context)!.test(url)

The fix adds a regexCache: Map<string, RegExp> inside proxyMiddleware that is populated when the proxy entries are initialized, and passes it down to doesProxyContextMatchUrl.

Test plan

• Existing proxy-related tests pass
• Dev server with server.proxy: { '^/api': '...' } routes requests correctly