feat: add to json(any): string conversion function

[rnewton-bp3]

Description

This PR adds a new built-in FEEL function to Json(object): string that performs the inverse of the existing from Json(json):any function. It allows FEEL expressions to serialize values (contexts, lists, primitives, temporal values, and durations) into JSON strings, similar to JSON.stringify(any) function in JavaScript. This is especially useful for integration scenarios where FEEL values need to be passed to systems expecting JSON.

Implementation details:

Introduced helper method convertToJsonValue(Val) to recursively map FEEL Val types into JSON-compatible Java/Scala values.

Supports all FEEL data types, including temporal values (ValDate, ValTime, ValDateTime, ValLocalTime, ValLocalDateTime) formatted using ISO-8601 standards.

Durations are serialized using their toString representation.

The resulting JSON string is wrapped in a ValString to be fully FEEL-compatible.

Added safety net to return ValError when serialization fails (though in practice all FEEL types are supported).

I could not create a meaningful test that triggers the ValError(s"Failed to convert object to JSON: ${obj.getClass.getSimpleName}") case because all FEEL data types are fully mapped in the convertToJsonValue(Val) function. Triggering this branch would require artificially passing in unsupported or non-FEEL types, which isn’t a real usage scenario.

Finally, I’m a complete Scala noob, so I appreciate any suggestions for making the code cleaner or more idiomatic.

Thanks to Alex Trukhny (@sashtran) 🙏 for the initial review.

Related issues
closes #602

[CLAassistant]

All committers have signed the CLA.

[saig0]

@rnewton-bp3 thank you for your contribution. 🎉 I'll review your changes soon. 👀

[Copilot]

Pull Request Overview

This PR adds a new built-in FEEL function to json(object): string that serializes FEEL values into JSON strings, providing the inverse functionality to the existing from json() function. This enables FEEL expressions to convert contexts, lists, primitives, temporal values, and durations into JSON format for integration scenarios.

• Implements to json() function with support for all FEEL data types including temporal values and durations
• Adds comprehensive test coverage for various data types and edge cases
• Uses ISO-8601 formatting for temporal values and toString representation for durations

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/main/scala/org/camunda/feel/impl/builtin/ConversionBuiltinFunctions.scala	Implements the to json() function with recursive value conversion logic
src/test/scala/org/camunda/feel/impl/builtin/BuiltinConversionFunctionsTest.scala	Adds comprehensive test cases covering all supported data types

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[saig0]

@rnewton-bp3 looks good. 👍 I have a few comments to improve and complete the PR. Please have a look. 🍪

[saig0]

@rnewton-bp3 looks very good. 👍

I have just one minor improvement. Please take a look. 🍰

[saig0]

@rnewton-bp3 looks great. 🎉

Thank you for contributing this valuable new feature. 🚀