fix: Exception on loop after agent call

[matheusandre1]

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What this PR does / why we need it: Fixes: #824

Special notes for reviewers: modified validations and I set it to optional.empty by default.

Additional information (if needed):

[Copilot]

Pull request overview

This PR fixes issue #824 by addressing a NullPointerException that occurred when looping after an agent call. The root cause was that ForTaskFunction.getForClass() and other Optional fields could return null, causing NPE when calling .isPresent() on them. The fix ensures all Optional fields are properly initialized and maintained as non-null throughout the object's lifecycle, including after serialization/deserialization.

Changes:

• Initialize Optional fields with Optional.empty() as defaults and add defensive normalization in constructor and deserialization
• Add readObject() method to handle deserialization safely
• Update JavaForExecutorBuilder to use safer Optional patterns (.map().orElse() instead of .isPresent())
• Add comprehensive regression tests covering initialization, serialization, and execution scenarios

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
experimental/types/src/main/java/io/serverlessworkflow/api/types/func/ForTaskFunction.java	Added default initialization of Optional fields, constructor, normalization logic, and readObject method for deserialization
experimental/lambda/src/main/java/io/serverlessworkflow/impl/executors/func/JavaForExecutorBuilder.java	Updated collectionFilterObject method to use safer Optional pattern
experimental/lambda/src/test/java/io/serverless/workflow/impl/executors/func/ForTaskFunctionRegressionTest.java	New test file with regression tests for Optional field initialization, serialization, and for-loop execution

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[fjtirado]

@matheusandre1 can you update the PR with my lastest comment in order to merge it?