fix: std.parseInt accepts + prefix like go-jsonnet (#1030)

## Summary

This PR updates `std.parseInt` to accept a leading `+` sign, matching
go-jsonnet behavior for decimal integer parsing.

## Behavior comparison

Local reference versions used:

- C++ jsonnet `v0.22.0`
- go-jsonnet `v0.21.0` and `v0.22.0`
- jrsonnet `0.5.0-pre99`

| Expression | C++ jsonnet | go-jsonnet | jrsonnet | sjsonnet before |
sjsonnet after |
| --- | --- | --- | --- | --- | --- |
| `std.parseInt("+1")` | error | `1` | error | error | `1` |
| `std.parseInt("+0")` | error | `0` | error | error | `0` |
| `std.parseInt("+01")` | error | `1` | error | error | `1` |
| `std.parseInt("+")` | error | error | error | error | error |
| `std.parseInt("+-1")` | error | error | error | error | error |
| `std.parseInt("-123")` | `-123` | `-123` | `-123` | `-123` | `-123` |
| `std.parseInt("123")` | `123` | `123` | `123` | `123` | `123` |

The intentional compatibility target here is go-jsonnet for
plus-prefixed decimal input. C++ jsonnet and jrsonnet currently reject
the leading `+` cases above.

## Changes

- `StringModule.scala`: handle a leading `+` sign in `std.parseInt`
- `parseInt_positive_sign.jsonnet`: golden regression coverage for
accepted `+` inputs and unchanged positive/negative baselines

## Tests

```bash
rtk ./mill 'sjsonnet.jvm[3.3.7]'.test.testOnly sjsonnet.FileTests
rtk git diff --check upstream/master...HEAD
```

Both passed locally after rebasing onto `upstream/master`.

Author: He-Pin

sjsonnet/src/sjsonnet/stdlib/StringModule.scala

@@ -1013,12 +1013,14 @@ object StringModule extends AbstractFunctionModule {
   private object ParseInt extends Val.Builtin1("parseInt", "str") {
     def evalRhs(str: Eval, ev: EvalScope, pos: Position): Val = {
       val s = str.value.asString
-      if (s.isEmpty || s == "-") {
+      if (s.isEmpty || s == "-" || s == "+") {
         Error.fail("Cannot parse '" + s + "' as an integer in base 10")
       }
       if (s.charAt(0) == '-') {
         val result = parseNat(s, 1, 10, "base 10")
         Val.cachedNum(pos, if (result == 0.0) 0.0 else -result)
+      } else if (s.charAt(0) == '+') {
+        Val.cachedNum(pos, parseNat(s, 1, 10, "base 10"))
       } else Val.cachedNum(pos, parseNat(s, 0, 10, "base 10"))
     }
   }

sjsonnet/test/resources/new_test_suite/parseInt_positive_sign.jsonnet

@@ -0,0 +1,8 @@
+// Test that std.parseInt accepts + prefix like go-jsonnet.
+assert std.parseInt("+123") == 123 : "parseInt(+123) should return 123";
+assert std.parseInt("+0") == 0 : "parseInt(+0) should return 0";
+assert std.parseInt("+007") == 7 : "parseInt(+007) should return 7";
+assert std.parseInt("+1") == 1 : "parseInt(+1) should return 1";
+assert std.parseInt("-123") == -123 : "parseInt(-123) should still work";
+assert std.parseInt("123") == 123 : "parseInt(123) should still work";
+true

sjsonnet/test/resources/new_test_suite/parseInt_positive_sign.jsonnet.golden

@@ -0,0 +1 @@
+true