docs: repair body-transformer SOAP example

[kayx23]

Summary

• pin the canonical Spring SOAP sample and a verified revision
• run the service in the APISIX quickstart Docker network
• update the SOAP namespace and upstream URI
• use the explicit base64-template contract
• return transformed responses with a JSON content type
• align the example with the surrounding plugin-reference structure

Why

The existing example no longer produces its documented result. The Spring repository moved, its SOAP namespace and service path changed, and localhost:8080 resolves inside the APISIX container. The route also leaves the transformed JSON response labeled as XML.

Validation

• verified the configuration against the APISIX 3.19.0 body-transformer, proxy-rewrite, and response-rewrite implementations
• verified Spring sample commit c9c88ea61cfbcef0de6275c7b6c8b1320549017a
• reproduced the previous example's 502 Bad Gateway
• sent a raw SOAP request to the pinned service successfully
• ran the documented JSON → SOAP XML → JSON round trip through apache/apisix:3.19.0-ubuntu
• verified HTTP 200, Content-Type: application/json, and the expected Spain response
• verified the explicit base64-template path
• ran markdownlint-cli@0.25.0, Vale on the changed section, and git diff --check

Companion documentation update api7/docs#2648 is merged.