Add abort support to FormChecker delayed checks (#26595)

* Add abort support to FormChecker delayed checks

Support optional AbortController/AbortSignal in FormChecker.delayedCheck, skip aborted queued checks, and handle aborted/failed requests without stalling queue processing. Add regression test covering cancellation of outdated checks.

* Update test/src/test/java/hudson/util/FormFieldValidatorTest.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Address review feedback on FormChecker abort handling

Guard non-GET URL parsing when no query string is present, prevent schedule queue stalls on synchronous sendRequest failures, and make the regression test wait for queue drain instead of fixed timing.

* Use JS based test instead of hacky htmlunit test

---------

Co-authored-by: Kevin <kevin@Kevins-MacBook-Pro-3.local>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Tim Jacomb <timjacomb1@gmail.com>

pom.xml

@@ -500,6 +500,16 @@ THE SOFTWARE.
                   <skip>${skip.frontend}</skip>
                 </configuration>
               </execution>
+              <execution>
+                <id>yarn test</id>
+                <goals>
+                  <goal>corepack</goal>
+                </goals>
+                <phase>test</phase>
+                <configuration>
+                  <arguments>yarn test</arguments>
+                </configuration>
+              </execution>
             </executions>
           </plugin>
           <plugin>