[EnvironmentCookbookVersionsEndpoint] Test sort_versions and filter_by_constraint

Add unit tests for the EnvironmentCookbookVersionsEndpoint helpers:
- sort_versions: descending semver sort, single element, empty, multi-digit
- filter_by_constraint: nil passthrough, exact/>=/>~ constraints, no match, immutability

Signed-off-by: David Crosby <dcrosby@fb.com>

Author: dafyddcrosby

spec/endpoints/environment_cookbook_versions_endpoint_spec.rb

@@ -0,0 +1,61 @@
+require "chef_zero/endpoints/environment_cookbook_versions_endpoint"
+
+describe ChefZero::Endpoints::EnvironmentCookbookVersionsEndpoint do
+  let(:server) { double("server") }
+  let(:endpoint) { described_class.new(server) }
+
+  describe "#sort_versions" do
+    it "returns versions sorted descending by semver" do
+      versions = ["1.0.0", "2.0.0", "1.5.0", "0.9.0"]
+      expect(endpoint.sort_versions(versions)).to eq(["2.0.0", "1.5.0", "1.0.0", "0.9.0"])
+    end
+
+    it "handles single-element lists" do
+      expect(endpoint.sort_versions(["1.0.0"])).to eq(["1.0.0"])
+    end
+
+    it "handles an empty list" do
+      expect(endpoint.sort_versions([])).to eq([])
+    end
+
+    it "sorts multi-digit version components numerically" do
+      versions = ["1.2.3", "1.10.0", "1.9.0"]
+      expect(endpoint.sort_versions(versions)).to eq(["1.10.0", "1.9.0", "1.2.3"])
+    end
+  end
+
+  describe "#filter_by_constraint" do
+    let(:versions) { { "apache" => ["1.0.0", "1.5.0", "2.0.0", "2.1.0"] } }
+
+    it "returns versions unchanged when constraint is nil" do
+      result = endpoint.filter_by_constraint(versions, "apache", nil)
+      expect(result["apache"]).to eq(["1.0.0", "1.5.0", "2.0.0", "2.1.0"])
+    end
+
+    it "filters with an exact version constraint" do
+      result = endpoint.filter_by_constraint(versions, "apache", "= 1.5.0")
+      expect(result["apache"]).to eq(["1.5.0"])
+    end
+
+    it "filters with a >= constraint" do
+      result = endpoint.filter_by_constraint(versions, "apache", ">= 2.0.0")
+      expect(result["apache"]).to eq(["2.0.0", "2.1.0"])
+    end
+
+    it "filters with a ~> constraint" do
+      result = endpoint.filter_by_constraint(versions, "apache", "~> 1.0")
+      expect(result["apache"]).to eq(["1.0.0", "1.5.0"])
+    end
+
+    it "returns empty array when no versions match" do
+      result = endpoint.filter_by_constraint(versions, "apache", "= 9.9.9")
+      expect(result["apache"]).to eq([])
+    end
+
+    it "does not mutate the original versions hash" do
+      original = versions.dup
+      endpoint.filter_by_constraint(versions, "apache", "= 1.0.0")
+      expect(versions).to eq(original)
+    end
+  end
+end