fix: ObjExtend should work with non-object lhs

[He-Pin]

Summary

Fix ObjExtend (lhs { body } syntax) to correctly handle non-object lhs values per jsonnet specification.

Problem

Per jsonnet spec, lhs { body } is equivalent to lhs + { body }. When lhs is not an object (e.g., string), sjsonnet was throwing a type error instead of falling back to add semantics.

Before:

"hello" { x: 1 }
// Error: Expected object, found string

After:

"hello" { x: 1 }
// Output: "hello{\"x\": 1}"

Solution

Modified visitObjExtend in Evaluator.scala to:

1. Check if base is Val.Obj - if so, use existing object extend logic
2. If base is not an object, evaluate body as standalone object (super = null)
3. Apply add semantics: string + object = string concat with stringified object

This matches:

• go-jsonnet: Desugars ApplyBrace to Binary + at parse time
• jrsonnet: Recent fix (commit b5fcc2639d) falls back to evaluate_add_op

Changes

• sjsonnet/src/sjsonnet/Evaluator.scala: Handle non-object lhs in visitObjExtend
• sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet: Test case
• sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet.golden: Expected output

Test Plan

• Added test case for string + object concatenation
• CI tests pass
• Verify no regression in existing object extend behavior