Cover prerelease pin via override

When the Gemfile dependency does not request a prerelease,
Resolver::Package's prerelease policy normally filters them out.
Because Definition#expanded_dependencies now feeds the effective
(override-applied) dep into Resolver::Base, an override that pins an
exact prerelease propagates into the package's prerelease decision
and the prerelease becomes selectable. Lock that contract in with an
integration test on a has_prerelease 1.0 / 1.1.pre fixture.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hsbt

spec/install/gemfile/override_spec.rb

@@ -50,5 +50,20 @@
 
       expect(the_bundle).to include_gems "myrack 0.9.1"
     end
+
+    it "pins a prerelease version that the Gemfile dependency would otherwise filter out" do
+      build_repo2 do
+        build_gem "has_prerelease", "1.0"
+        build_gem "has_prerelease", "1.1.pre"
+      end
+
+      install_gemfile <<-G
+        source "https://gem.repo2"
+        override "has_prerelease", version: "= 1.1.pre"
+        gem "has_prerelease"
+      G
+
+      expect(the_bundle).to include_gems "has_prerelease 1.1.pre"
+    end
   end
 end