Add comprehensive test coverage for add operator's type handling

[Copilot]

The add operator now supports arrays, objects, and throws errors on incompatible types, but lacked test coverage for these branches.

Changes

• String handling tests: Non-numeric string concatenation and numeric string addition
• Array concatenation tests: Basic and empty array cases
• Object merge tests: Property merging and overlapping key behavior
• Type coercion tests: Boolean to number conversion (true + 1 = 2)
• Error handling tests: Incompatible type combinations throw descriptive errors
• Edge case tests: null + null behavior

Example

parser.evaluate('[1, 2] + [3, 4]')           // [1, 2, 3, 4]
parser.evaluate('{a: 1} + {b: 2}')           // {a: 1, b: 2}
parser.evaluate('"hello" + "world"')         // "helloworld"
parser.evaluate('"5" + "3"')                 // 8
parser.evaluate('5 + [1, 2]')                // Error: Cannot add values of incompatible types

All branches of the add function now have test coverage.

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[Copilot]

Pull request overview

This PR adds comprehensive test coverage for the add operator's type handling capabilities, ensuring all code branches are tested including string concatenation, array concatenation, object merging, type coercion, and error handling.

Changes:

• Added 7 new test cases covering string handling, array operations, object merging, type coercion, null handling, and error scenarios
• All previously untested branches of the add operator now have test coverage

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