feat(middleware): implement FixedWindow rate limiter and enhance time…

[KARTIKrocks]

…outWriter functionality

Summary

Changed

• middleware — Renamed TokenBucket → FixedWindow and NewTokenBucket → NewFixedWindow to accurately reflect the fixed-window counter algorithm; default limiter in RateLimit() updated accordingly

Added

• middleware — timeoutWriter.Unwrap() returns the underlying http.ResponseWriter, enabling http.Flusher/http.Hijacker detection via http.ResponseController
• middleware — timeoutWriter.Flush() implements http.Flusher with timeout-aware guarding, allowing SSE and chunked responses through the timeout middleware
• middleware — Doc comment on Timeout clarifying goroutine leak behavior when a handler does not respect context cancellation

Removed

• middleware — TokenBucket and NewTokenBucket (replaced by FixedWindow and NewFixedWindow)

Checklist

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