fix: apply addition semantics to object extension

[He-Pin]

Motivation

Jsonnet defines lhs { body } as syntactic sugar for lhs + { body }. Previously, sjsonnet required an object on the left and rejected valid expressions such as "hello" { x: 1 } with Expected object, found string. This expression should return "hello{\"x\": 1}", just like explicit addition.

Modification

• Preserve the existing object-inheritance fast path, including self, super, assertions, and lazy fields.
• For a non-object left operand, delegate to the existing BinaryOp + evaluator using the already evaluated value. This evaluates the original left expression once and shares object construction, stringification, and error reporting with explicit addition.
• Add regression coverage for all left-operand types, static and dynamic object bodies, comprehensions, hidden fields, evaluation order, lazy errors, inheritance, shared object-body caches, and the std.foldl fallback. Include executable number/array error goldens and 14 success assertions.

Result

String/object extension now follows normal addition semantics. Unsupported left-operand types still fail, with the same diagnostics as +. The existing strict-mode restriction on consecutive object bodies is preserved.

Validation completed locally:

• JVM Scala 2.12.21, 2.13.18, and 3.3.8: 677/677 tests passed per version (2,031 total).
• Formatting checks passed for all JVM Scala versions; the Scala 3.3.8 assembly built successfully.
• The assembled CLI matched the success, number-error, and array-error golden files, including exit codes and diagnostics.
• The 14 success assertions passed on Go Jsonnet 0.21.0 and 0.22.0.

./mill --no-daemon -j 1 'sjsonnet.jvm[_].test' + 'sjsonnet.jvm[_].checkFormat' + 'sjsonnet.jvm[3.3.8].assembly'

JS, WASM, Native, and JMH were not run locally.

References

• Jsonnet specification: desugaring
• Corresponding jrsonnet fix