test: Remove Jackson dependency from petclinic fixture

The upstream spring-framework-petclinic project changed Jackson
coordinates from com.fasterxml.jackson to tools.jackson.core,
breaking our custom ShowAvailable controller in petclinic-fw tests.

Replaced ObjectMapper with manual JSON formatting and removed the
now-unused Result inner class, making the test fixture resilient
to upstream dependency changes.

Author: dividedmind

agent/src/test/fixture/shared/src/main/java/org/springframework/samples/petclinic/web/ShowAvailable.java

@@ -1,34 +1,19 @@
 package org.springframework.samples.petclinic.web;
 
-import java.io.IOException;
-
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RestController;
 
-import com.fasterxml.jackson.databind.ObjectMapper;
-
 /**
  * ShowAvailable is a simple Spring Controller that simply returns the number
  * of bytes available in the request's InputStream.
  */
 @RestController
 public class ShowAvailable {
-  private static class Result {
-    private final int available;
-
-    public Result(int available) {
-      this.available = available;
-    }
-
-    public String getAvailable() {
-      return Integer.toString(available);
-    }
-  }
 
   @PostMapping("/showavailable")
   public String doPost(@RequestBody String body) throws Exception {
-    return new ObjectMapper().writeValueAsString(new Result(body.length()));
+    return String.format("{\"available\":\"%d\"}", body.length());
   }
 
 }