[2.x] fix: Sequence server start before publish in ivyless HTTP scripted tests

The ivyless-publish-http and ivyless-publish-http-plugin scripted tests
defined publishToHttp as three unordered `.value` dependencies
(startPublishServer, publish, stopPublishServer). sbt evaluates task
dependencies before the task body in an unspecified order, so publish
could run before the HTTP server was listening and fail intermittently
with "Connection refused". It is reproducible under CPU contention (a
2-vCPU runner), which is why it began flaking on the JDK-25 shard and
turned develop red.

Use Def.sequential to guarantee start -> publish -> stop ordering, the
way the sibling ivyless-publish-maven-http test already sequences via
separate scripted commands.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

sbt-app/src/sbt-test/dependency-management/ivyless-publish-http-plugin/build.sbt

@@ -38,11 +38,7 @@ stopPublishServer := {
 }
 
 val publishToHttp = taskKey[Unit]("Publish to HTTP server (start server, publish, stop server)")
-publishToHttp := {
-  startPublishServer.value
-  try publish.value
-  finally stopPublishServer.value
-}
+publishToHttp := Def.sequential(startPublishServer, publish, stopPublishServer).value
 
 val checkPublished = taskKey[Unit]("Check ivyless publish wrote the plugin cross-suffix PUT path")
 checkPublished := {

sbt-app/src/sbt-test/dependency-management/ivyless-publish-http/build.sbt

@@ -35,11 +35,7 @@ stopPublishServer := {
 }
 
 val publishToHttp = taskKey[Unit]("Publish to HTTP server (start server, publish, stop server)")
-publishToHttp := {
-  startPublishServer.value
-  try publish.value
-  finally stopPublishServer.value
-}
+publishToHttp := Def.sequential(startPublishServer, publish, stopPublishServer).value
 
 val checkIvylessPublish = taskKey[Unit]("Check that ivyless publish produced the expected files")
 checkIvylessPublish := {