This commit fixes several critical memory leaks in the gzip compression
middleware that could lead to significant memory accumulation under
specific usage patterns.

Changes:
1. Fixed WebSocket/Hijack resource leak (compress.go:213-219)
   - Close gzip writer before hijacking connection
   - Prevents ~32KB leak per WebSocket connection
   - Critical for long-lived WebSocket connections

2. Fixed Flush() buffer accumulation (compress.go:200-204)
   - Clear buffer after successful write during Flush()
   - Prevents unbounded memory growth in SSE scenarios
   - Important for Server-Sent Events and streaming responses

3. Improved pool management (compress.go:138-149)
   - Check writer state before returning to pool
   - Prevent corrupted writers from being reused
   - Eliminates potential data corruption issues

Impact:
- WebSocket connections: no longer leak gzip writers (~32KB each)
- SSE/streaming: prevents linear buffer growth
- Pool safety: eliminates race conditions from reused writers

Fixes potential memory leaks affecting:
- WebSocket applications using compression middleware
- Server-Sent Events endpoints
- Long-lived streaming connections
- High-concurrency scenarios