fix: select Spring 6 REST adapter for samples

[SavitarC]

What changed

• add an opt-in dubbo-rest-spring6 Maven profile to the Spring MVC Triple REST sample
• add the same profile to the Spring Security resource-server sample
• when enabled with -Ddubbo.rest.spring6=true, explicitly select:
  • dubbo-spring-boot-3-autoconfigure
  • dubbo-rest-spring6

Why

apache/dubbo#16394 separates the Spring 5 and Spring 6 REST adapters and removes the legacy dubbo-rest-spring implementation from the aggregate dubbo JAR.

These Spring Boot 3 samples use Spring MVC mappings, so they must explicitly choose the Spring 6 / Jakarta adapter when tested against the candidate Dubbo build.

Compatibility

The profile is disabled by default. Normal sample builds therefore remain compatible with the current released Dubbo baseline, which does not provide dubbo-rest-spring6.

After a Dubbo release containing dubbo-rest-spring6 becomes the samples baseline, these dependencies can become unconditional and the temporary activation property can be removed.

The profile selects only dubbo-rest-spring6; it does not add the legacy dubbo-rest-spring adapter.

Validation

With apache/dubbo#16394 at ea51d470b1 and -Ddubbo.version=3.3.7-SNAPSHOT -Dspring.version=6.1.5 -Ddubbo.rest.spring6=true:

• the Spring MVC sample dependency tree contains dubbo-spring-boot-3-autoconfigure and dubbo-rest-spring6, with no legacy adapter
• the Spring Security resource-server dependency tree contains the same explicit Spring 6 composition, with no legacy adapter
• the Spring MVC provider registered 48 REST mappings and all 36 ConsumerIT tests passed
• the Spring Security provider registered its Spring MVC REST mappings and both ResourceServerIT tests passed
• with the profile disabled, the packaged Spring MVC sample contains neither REST Spring adapter and /demo/hello returns 404 Invoker not found
• all current checks for this PR pass; those checks intentionally leave the profile disabled because the released Dubbo baseline does not provide dubbo-rest-spring6
• git diff --check passed

CI coordination

This PR depends on the new artifact from apache/dubbo#16394. Conversely, the samples workflow in apache/dubbo#16394 currently checks out dubbo-samples@master and does not enable this opt-in profile. Merging this PR unchanged by itself therefore does not make the core PR's samples checks green; the cross-repository ref/profile activation or merge sequence must be coordinated separately.

Related

• depends on feat: add Spring 6 REST adapter artifact dubbo#16394