[Bug] Deal with list arguments that are provided by LLM without brackets

Description

As some kind of hallucination - LLM may rarely provide list arguments as comma-separated elements without the required brackets wrapping them.

Example (happened in the test test_that_a_tool_with_a_parameter_attached_to_a_choice_provider_gets_the_tool_context):

How to Reproduce

This bug is hard to preproduce as it depends on non-deterministic LLM behavior.

Expected Behavior

This specific bug happens in _validate_argument_value. At this place in code, the arguments types are known, so we can safely split the list even without the brackets, as long as the list delimiter (comma) is correct.

Environment

• OS: iOS
• Python version 3.10.17
• Parlant version

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