fix: preserve query string params for multipart requests in Gateway MVC

[ar249]

Problem

When a multipart POST request with query string parameters was routed through Spring Cloud Gateway MVC, the query string parameters were silently dropped before reaching the downstream service.

Root cause

StandardMultipartHttpServletRequest merges multipart body parts into the parameter map, overwriting query string parameters in the process.

Solution

Updated the logic inside getParameterMap() in GatewayMultipartHttpServletRequest to decode and return only the query string parameters for gateway requests using MvcUtils.decodeQueryString(), ensuring the multipart body is not parsed and query string params are preserved as-is.

Changes

• GatewayMultipartHttpServletReques.getParameterMap() — fixed query string decoding logic inside the gateway request guard block

• GatewayMvcMultipartResolverTest — new test class covering:

  • Multiple distinct query string params are decoded correctly
  • null query string returns an empty map
  • Empty query string returns an empty map
  • Multipart body is not parsed for gateway requests (initializeMultipart() is never called, body fields do not appear in the parameter map)
  • Non-gateway requests delegate to super.getParameterMap()

• Fixes gateway MVC - Multi-part + query string not working #3220