Update smithy-rules-engine-tests with new std lib functions and bdd-based tests

[landonxjames]

Background

• What do these changes do?
• Why are they important?

Add new tests for the new coalesce, split, and ite std lib functions.

Work to automate adding the endpointBdd trait to these test models is tracked in #2949.

Testing

• How did you test these changes?

I believe these are tested on build so will be tested by CI. Also tested these against the Rust test generator.

Links

• Links to additional context, if necessary
• Issue #, if applicable (see here for a list of keywords to use for linking issues)

---

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

[github-actions]

This pull request does not contain a staged changelog entry. To create one, use the ./.changes/new-change command. For example:

./.changes/new-change --pull-requests "#2945" --type feature --description "Update smithy-rules-engine-tests with new std lib functions and bdd-based tests"

Make sure that the description is appropriate for a changelog entry and that the proper feature type is used. See ./.changes/README or run ./.changes/new-change -h for more information.

[landonxjames]

Capturing offline discussion with @sugmanue:

• Use the same namespace for all of the tests namespace smithy.rules.tests
• Combine the bdd-tests and tree-tests into a single model for each std-lib fn that contains both traits
• Eventually will be better if the endpointBdd trait can just be provided as a transformation/part of the model projection so that we don't have to worry about manually compiling it and keeping it in sync (@sugmanue to own this part)