[BUG] useFeignClientContextId default option causes IllegalStateException at runtime

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Following the introduction of useFeignCleintContextId with default true in #20943, the contextId by default fails to resolve a valid service name and throws an exception, e.g.:

Service id not legal hostname (${countries.name})
java.lang.IllegalStateException: Service id not legal hostname (${countries.name})
	at org.springframework.util.Assert.state(Assert.java:79)
	at org.springframework.cloud.openfeign.FeignClientsRegistrar.getName(FeignClientsRegistrar.java:111)
	at org.springframework.cloud.openfeign.FeignClientsRegistrar.getContextId(FeignClientsRegistrar.java:346)
	at org.springframework.cloud.openfeign.FeignClientsRegistrar.eagerlyRegisterFeignClientBeanDefinition(FeignClientsRegistrar.java:230)
	at org.springframework.cloud.openfeign.FeignClientsRegistrar.registerFeignClient(FeignClientsRegistrar.java:215)
	at org.springframework.cloud.openfeign.FeignClientsRegistrar.registerFeignClients(FeignClientsRegistrar.java:204)
	at org.springframework.cloud.openfeign.FeignClientsRegistrar.registerBeanDefinitions(FeignClientsRegistrar.java:154)
	at org.springframework.context.annotation.ImportBeanDefinitionRegistrar.registerBeanDefinitions(ImportBeanDefinitionRegistrar.java:86)

This is with org.springframework.cloud:spring-cloud-starter-openfeign:4.3.0

The generated feign client looks like this:

@FeignClient(name="${countries.name:countries}", contextId="${countries.contextId:${countries.name}}",  configuration = ClientConfiguration.class)
public interface CountriesApiClient extends CountriesApi {
}

openapi-generator version

7.13.0

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

#20943

Suggest a fix

Is it possible for the contextId to actually resolve the nested countries.name expression or is it doomed to fail? In any case, I think it would be better to default this new option to false rather than true.

Start by reviewing the useFeignClientContextId change in #20943 and reproduce the generated CountriesApiClient with Spring Cloud OpenFeign 4.3.0. Trace how the name and nested contextId expressions are produced, then add or update a regression test so the default configuration no longer causes the reported IllegalStateException.