Fix agent JAR lookup

[adinauer]

📜 Description

#skip-changelog

Fixes CI to look for the version of Java agent we actually want to test (specified in gradle.properties).

💡 Motivation and Context

Tests were failing most likely due to cached builds on CI that contained both 8.18.0 and 8.19.0 for the agent JAR and it kept using 8.18.0. This then caused mixed version detection to kick in and fail Spring Boot startup on the E2E tests.

💚 How did you test it?

📝 Checklist

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[adinauer]

• Fix agent JAR lookup #4633 👈 (View in Graphite)
• main

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