Re-apply Jackson 2.18.6 upgrade with round-trip test coverage

[darklight3it]

Issue #, if available:

Related to #594

Description of changes:

Upgrading Jackson in the serialization package from 2.15.4 to 2.18.6 . The previous update was reverted because of a suspected regression on aws-lambda-serialization here.

The root cause of the failure was not in the published artifact. I confirmed this running the new suite of tests introduced in #598 against the 1.3.0 version of aws-lambda-serialization.

cd aws-lambda-java-tests  
mvn clean test

[INFO] Results:
[INFO] 
[INFO] Tests run: 125, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  3.117 s
[INFO] Finished at: 2026-03-26T12:10:43Z

The real root cause was in aws-lambda-java-serializzation.yml, specifically in:

mvn -B package install --file aws-lambda-java-serialization/pom.xml

Both package and install execute the shade goal from maven-shade-plugin, responsible for avoiding dependency collision with user code.
This plugin executes a non idempotent action in DateTimeModule. Specifically moving the registering namespace in the following way:

"com.amazonaws.lambda.unshade.thirdparty.org.joda.time.DateTime"
                    │
                    │ rule: unshade.thirdparty.org.joda.time → org.joda.time
                    ▼
         "org.joda.time.DateTime"
                    │
                    │ rule: org.joda.time → com.amazonaws.lambda.thirdparty.org.joda.time
                    ▼
"com.amazonaws.lambda.thirdparty.org.joda.time.DateTime"

Thus not registering the correct event imported from the aws-lambda-java-events library. The tests are now failing because the default Joda Time jackson serializer changed the constructor signature.

Target (OCI, Managed Runtime, both):

Both

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.