test(pkg): preserve platform-specific rejection reasons

test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-platform-rejection-reason.t

@@ -0,0 +1,40 @@
+Solver diagnostics evaluate package availability in the failing platform's
+environment.
+
+  $ mkrepo
+  $ add_mock_repo_if_needed
+
+  $ mkpkg foo <<'EOF'
+  > available: os != "linux"
+  > EOF
+
+  $ cat >dune-workspace <<EOF
+  > (lang dune 3.20)
+  > (repository
+  >  (name mock)
+  >  (url "file://$(pwd)/mock-opam-repository"))
+  > (lock_dir
+  >  (repositories mock)
+  >  (solve_for_platforms
+  >   ((arch x86_64)
+  >    (os linux))))
+  > (pkg enabled)
+  > EOF
+  $ write_portable_lockdirs_project
+
+The package exists in the repository but is unavailable on Linux. The rejection
+reason must be derived from Linux rather than a platform-less environment.
+
+  $ dune pkg lock
+  Error:
+  Unable to solve dependencies while generating lock directory: dune.lock
+
+  The dependency solver failed to find a solution for the following platforms:
+  - arch = x86_64; os = linux
+  ...with this error:
+  Couldn't solve the package dependency formula.
+  Selected candidates: x.dev
+  - foo -> (problem)
+      No usable implementations:
+        foo.0.0.1: Availability condition not satisfied
+  [1]