FINERACT-2534: Add unit tests for StreamUtil utility class

[devi-pathak2263]

Description

This PR adds unit tests for the StreamUtil utility class in fineract-core.

The following methods are covered:

• foldLeft collector functionality
• mergeMapsOfLists collector functionality

These tests verify that:

• foldLeft correctly reduces a stream using the provided accumulator.
• mergeMapsOfLists correctly merges maps with list values by concatenating lists for identical keys.

This improves test coverage and ensures the correctness of commonly used utility collectors.

JIRA: https://issues.apache.org/jira/browse/FINERACT-2534

##Testing

Executed locally:
./gradlew spotlessApply
./gradlew :fineract-core

Checklist

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• Follow our [coding conventions](https://cwiki.apache.org/confluence/display/FINERACT/Coding+Conventions).
• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes
• [This PR must not be a "code dump"](https://cwiki.apache.org/confluence/display/FINERACT/Pull+Request+Size+Limit). Large changes can be made in a branch, with assistance. Ask for help on the [developer mailing list](https://fineract.apache.org/#contribute).

[adamsaghy]

@devi-pathak2263 Please squash your commits.

[devi-pathak2263]

@devi-pathak2263 Please squash your commits.

Thanks for the feedback. I have squashed the commits into a single commit.

[adamsaghy]

@devi-pathak2263 Can you please:

• Set the commit email address to match with your github account email address?
• Set GPG signature for commit?

[devi-pathak2263]

@devi-pathak2263 Can you please:

• Set the commit email address to match with your github account email address?
• Set GPG signature for commit?

I have now:

• updated the commit author email to match my GitHub account
• amended the commit with a GPG signature

The latest commit should now be verified. Let me know if anything else needs improvement.