[WebFlux] XForwardedHeadersFilter emits overlapping X-Forwarded-Prefix values for chained path filters

Describe the bug

When a WebFlux route applies more than one StripPrefix filter, Spring Cloud Gateway sends overlapping X-Forwarded-Prefix values to the downstream service.

For a request to /tenant/api/blue that is stripped to /blue, the gateway sends:

X-Forwarded-Prefix: /tenant/api,/api

A downstream Spring application with forwarded-header processing enabled combines those values and reconstructs /tenant/api/api as the context path instead of /tenant/api.

The client request in this example does not contain an X-Forwarded-Prefix header. Both values are produced by a single gateway while processing one route.

How to reproduce

Run the gateway on port 8080 with this route:

server:
  port: 8080

spring:
  cloud:
    gateway:
      server:
        webflux:
          trusted-proxies: ".*"
          routes:
            - id: chained-strip-prefix
              uri: http://localhost:8081
              predicates:
                - Path=/tenant/api/**
              filters:
                - StripPrefix=1
                - StripPrefix=1

Run a downstream Spring WebFlux application on port 8081 with forwarded-header processing enabled:

server:
  port: 8081
  forward-headers-strategy: framework

Add an endpoint that returns the path information seen by the downstream application:

@RestController
class PathController {

	@GetMapping("/blue")
	Map<String, String> path(ServerHttpRequest request) {
		RequestPath path = request.getPath();
		return Map.of(
				"contextPath", path.contextPath().value(),
				"pathWithinApplication", path.pathWithinApplication().value());
	}

}

Send a request through the gateway:

curl http://localhost:8080/tenant/api/blue

The configured filters change the request path as follows:

/tenant/api/blue
    -> /api/blue
    -> /blue

Actual behavior

The downstream request contains overlapping prefix values:

X-Forwarded-Prefix: /tenant/api,/api

The downstream application reports a duplicated context-path segment:

{
  "contextPath": "/tenant/api/api",
  "pathWithinApplication": "/blue"
}

Expected behavior

The gateway should send the external prefix removed from the final routed path once:

X-Forwarded-Prefix: /tenant/api

The downstream application should therefore report:

{
  "contextPath": "/tenant/api",
  "pathWithinApplication": "/blue"
}

Version

spring-cloud-gateway-server-webflux: 5.0.3-SNAPSHOT

Start at the WebFlux XForwardedHeadersFilter entry point and reproduce the chained StripPrefix route described in the issue. Add coverage for the /tenant/api/blue request and verify that the downstream request contains one X-Forwarded-Prefix value, /tenant/api, with the expected context path.