Test downstream without explicit traits (rely on FoundationSupport default)

Explicit traits: ["FoundationSupport"] may trigger a SPM 6.2 bug where
product dependencies of trait-gated sub-targets aren't included in the
build plan. Since FoundationSupport is already the default trait, omitting
the explicit specification should produce identical behavior while avoiding
the bug.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Joannis

.github/workflows/test.yml

@@ -122,7 +122,7 @@ jobs:
           let package = Package(
             name: "DownstreamTestNoNIO",
             dependencies: [
-              .package(path: "../IkigaJSON", traits: ["FoundationSupport"]),
+              .package(path: "../IkigaJSON"),
             ],
             targets: [
               .executableTarget(