Improve test assertion in ValidatorRouteTest

[apupier]

use AssertJ to have more details in case of test failures

Description

Target

• I checked that the commit is targeting the correct branch (Camel 4 uses the main branch)

Tracking

• If this is a large change, bug fix, or code improvement, I checked there is a JIRA issue filed for the change (usually before you start working on it).

Apache Camel coding standards and style

• I checked that each commit in the pull request has a meaningful subject line and body.

• I have run mvn clean install -DskipTests locally from root folder and I have committed all auto-generated changes.

[github-actions]

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[apupier]

requires #23289 to pass but at least it shows the better error message

[github-actions]

🧪 CI tested the following changed modules:

• components/camel-json-validator

✅ POM dependency changes: targeted tests included

Modules affected by dependency changes (1)

• :camel-json-validator

All tested modules (9 modules)

• Camel :: AI :: LangChain4j :: Agent
• Camel :: JBang :: MCP
• Camel :: JBang :: Plugin :: Route Parser
• Camel :: JBang :: Plugin :: TUI
• Camel :: JBang :: Plugin :: Validate
• Camel :: JSON validator
• Camel :: Launcher :: Container
• Camel :: YAML DSL :: Validator
• Camel :: YAML DSL :: Validator Maven Plugin

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