Problem

Close #3978. When Spring Cloud Gateway uses CircuitBreaker with fallback, if the backend returns a response with chunked transfer encoding, the connection to the backend is not properly released, causing a connection pool leak. Eventually, the connection pool becomes exhausted, and subsequent requests fail with 504 GATEWAY_TIMEOUT due to PoolAcquireTimeout.

Root Cause

The issue is in ServerWebExchangeUtils.reset() method. When CircuitBreaker triggers fallback, it calls reset(exchange) to prepare the exchange for the fallback request. However, the current implementation only:

1. Removes added response headers
2. Clears the alreadyRouted flag

It does not release the client connection stored in CLIENT_RESPONSE_CONN_ATTR, which was established by NettyRoutingFilter to the backend.

Connection leak flow:

Request → NettyRoutingFilter creates connection → stored in CLIENT_RESPONSE_CONN_ATTR
    ↓
Backend returns 500 + chunked response
    ↓
CircuitBreaker triggers fallback
    ↓
reset(exchange) is called, but connection is NOT released
    ↓
Fallback request uses a new connection
    ↓
Original connection leaks forever → pool exhaustion

Solution

Enhance ServerWebExchangeUtils.reset() to properly dispose the client connection