fix(request,response): cache regexp patterns, set Content-Length, and…

[KARTIKrocks]

… validate JSONP callbacks

Summary

Changed

• request — MatchesRegexp now caches compiled *regexp.Regexp patterns in a sync.Map, eliminating repeated recompilation
• request — Validation.MatchesPattern delegates to the cached MatchesRegexp instead of calling regexp.MatchString directly
• response — write() marshals JSON to a buffer first and sets the Content-Length header before writing, improving client framing behavior
• response — JSONP output is now prefixed with /**/ to mitigate Rosetta Flash and content-type-sniffing attacks

Fixed

• response — JSONP callback parameter is validated against ^[a-zA-Z_$][a-zA-Z0-9_$.]*$; invalid names return 400 to prevent XSS injection
• response — JSON encoding errors in write() are now caught before headers are sent, returning a clean 500 instead of a truncated body

Checklist

• fmt, vet, lint, test, build passes (make all)
• New code has tests where appropriate
• Breaking changes are documented