fix stepper issues

[CATISNOTSODIUM]

Description

Fixing new stepper (tracer) related issues. This includes

• Primitive math expression does not work in the current stepper.
• String concatenation visual bug. ("a" + "b" should be "ab", not ab)

Type of change

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code quality improvements

How to test

You can link this package with source academy frontend and test the new stepper playground (refer to this pr source-academy/frontend#3125).

• Primitive math expression does not work in the current stepper.
  Math-related function calls (e.g. math_sqrt(4)) must not lead to evaluation stuck message if the argument type (number) is correctly specified.
• String concatenation visual bug. ("a" + "b" should be "ab", not ab)

Checklist

• I have tested this code
• I have updated the documentation

[coveralls]

Pull Request Test Coverage Report for Build 15602932475

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 77.062%

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Totals
Change from base Build 15600110367:	0.01%
Covered Lines:	10100
Relevant Lines:	12737

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💛 - Coveralls

[martin-henz]

Looks good!