Fix memory leaks in compress middleware

[spaumx]

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

[aldas]

please provide proper description of the problem. it takes a little to much effort to understand the real-world situation, and you already have LLM created bullet list here so it should not be a problem.

p.s. tests/poc would be also helpful.