Warn about missing JSON extension when using REST Client

[geoand]

Fixes: #35586

[cescoffier]

2 very minor comments

[quarkus-bot]

Failing Jobs - Building c7767a2

Status	Name	Step	Failures	Logs	Raw logs	Build scan
✖	JVM Tests - JDK 11	Build	⚠️ Check →	Logs	Raw logs
✔️	JVM Tests - JDK 17
✖	JVM Tests - JDK 17 Windows	Build	Failures	Logs	Raw logs
✔️	JVM Tests - JDK 20
✔️	Maven Tests - JDK 11
✖	Maven Tests - JDK 11 Windows	Build	Failures	Logs	Raw logs

Full information is available in the Build summary check run.

Failures

⚙️ JVM Tests - JDK 17 Windows #

- Failing: extensions/vertx/deployment 
! Skipped: devtools/cli extensions/agroal/deployment extensions/amazon-lambda-http/deployment and 379 more

📦 extensions/vertx/deployment

✖ io.quarkus.vertx.mdc.VertxMDCTest.mdc line 144 - More details - Source on GitHub

org.opentest4j.AssertionFailedError: expected: <10> but was: <0>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)

✖ io.quarkus.vertx.mdc.VertxMDCTest.mdcNonVertxThreadTest line 121 - More details - Source on GitHub

java.lang.AssertionError: 

Expected: a collection containing "Test MDC value ### Test 1"

---

⚙️ Maven Tests - JDK 11 Windows #

📦 integration-tests/maven

✖ io.quarkus.maven.it.JarRunnerIT.testPlatformPropertiesOverridenInApplicationProperties line 135 - More details - Source on GitHub

org.awaitility.core.ConditionTimeoutException: Condition with lambda expression in io.quarkus.maven.it.JarRunnerIT that uses io.quarkus.maven.it.verifier.MavenProcessInvocationResult was not fulfilled within 1 minutes.
	at org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
	at org.awaitility.core.CallableCondition.await(CallableCondition.java:78)

✖ io.quarkus.maven.it.JarRunnerIT.testPlatformPropertiesOverridenInApplicationProperties line 135 - More details - Source on GitHub

org.awaitility.core.ConditionTimeoutException: Condition with lambda expression in io.quarkus.maven.it.JarRunnerIT that uses io.quarkus.maven.it.verifier.MavenProcessInvocationResult was not fulfilled within 1 minutes.
	at org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
	at org.awaitility.core.CallableCondition.await(CallableCondition.java:78)