fix(code-execution): auto-wrap multi-line function calls, serialize results as JSON

[rabi]

Summary

The code execution extension's result wrapping logic only looked at the last line of code, not the complete multi-line statement. When code ended with continuation characters like });, the wrapping produced invalid JavaScript syntax like __result__ = })), causing parse errors and infinite retry loops.

• Extract wrap_code_for_result() to handle multiline expressions ending with });
• Detect continuation lines (closing brackets, method chaining)
• Find expression start for proper wrapping instead of wrapping interior lines
• Serialize results using to_json() instead of display() to show actual array/object contents
• Update LLM instructions for clearer code formatting guidance

Type of Change

• Feature
• Bug fix
• Refactor / Code quality
• Performance improvement
• Documentation
• Tests
• Security fix
• Build / Release
• Other (specify below)

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

Unit tests and manual testing.

Related Issues

Closes: #6266

[alexhancock]

thanks for looking at this

the result wrapping/capture was always one of my least favorite parts of what I originally merged. let me take a look at a more comprehensive fix that would avoid the need for something like this. if I don't come up with anything in a day or two we can proceed with this

[alexhancock]

#6343

[rabi]

Closing in favor of #6343