Add comprehensive language_version_manager tests

Tests verify correct Python version selection behavior:
- Lowest compatible version for range constraints
- Correct handling of exact versions
- Default to highest when no version specified
- Support for both PEP 621 and Poetry formats

Co-authored-by: AbhishekBhaskar <22154418+AbhishekBhaskar@users.noreply.github.com>

Author: Copilot

python/spec/dependabot/python/language_version_manager_spec.rb

@@ -0,0 +1,154 @@
+# typed: false
+# frozen_string_literal: true
+
+require "spec_helper"
+require "dependabot/python/language_version_manager"
+require "dependabot/python/file_parser/python_requirement_parser"
+require "dependabot/dependency_file"
+
+RSpec.describe Dependabot::Python::LanguageVersionManager do
+  let(:manager) { described_class.new(python_requirement_parser: parser) }
+  let(:parser) { Dependabot::Python::FileParser::PythonRequirementParser.new(dependency_files: files) }
+
+  describe "#python_version" do
+    subject(:python_version) { manager.python_version }
+
+    context "with pyproject.toml containing requires-python" do
+      let(:files) { [pyproject_file] }
+      let(:pyproject_file) do
+        Dependabot::DependencyFile.new(
+          name: "pyproject.toml",
+          content: pyproject_content
+        )
+      end
+
+      context "when requires-python specifies a range" do
+        let(:pyproject_content) do
+          <<~TOML
+            [project]
+            name = "test-package"
+            requires-python = ">= 3.9, <3.13"
+          TOML
+        end
+
+        it "selects the lowest compatible Python version" do
+          expect(python_version).to eq("3.9.24")
+        end
+      end
+
+      context "when requires-python specifies minimum only" do
+        let(:pyproject_content) do
+          <<~TOML
+            [project]
+            name = "test-package"
+            requires-python = ">= 3.10"
+          TOML
+        end
+
+        it "selects the lowest compatible Python version" do
+          expect(python_version).to eq("3.10.19")
+        end
+      end
+
+      context "when requires-python specifies exact version" do
+        let(:pyproject_content) do
+          <<~TOML
+            [project]
+            name = "test-package"
+            requires-python = "3.11"
+          TOML
+        end
+
+        it "selects Python 3.11" do
+          expect(python_version).to eq("3.11.14")
+        end
+      end
+
+      context "with Poetry format python dependency" do
+        let(:pyproject_content) do
+          <<~TOML
+            [tool.poetry.dependencies]
+            python = "^3.12"
+          TOML
+        end
+
+        it "selects the lowest compatible Python version" do
+          expect(python_version).to eq("3.12.12")
+        end
+      end
+
+      context "without Python version specified" do
+        let(:pyproject_content) do
+          <<~TOML
+            [project]
+            name = "test-package"
+          TOML
+        end
+
+        it "defaults to the highest available Python version" do
+          expect(python_version).to eq("3.14.0")
+        end
+      end
+    end
+
+    context "with no dependency files" do
+      let(:files) { [] }
+
+      it "defaults to the highest available Python version" do
+        expect(python_version).to eq("3.14.0")
+      end
+    end
+  end
+
+  describe "#python_requirement_string" do
+    subject(:requirement_string) { manager.python_requirement_string }
+
+    context "with pyproject.toml containing requires-python" do
+      let(:files) { [pyproject_file] }
+      let(:pyproject_file) do
+        Dependabot::DependencyFile.new(
+          name: "pyproject.toml",
+          content: <<~TOML
+            [project]
+            name = "test-package"
+            requires-python = ">= 3.9, <3.13"
+          TOML
+        )
+      end
+
+      it "returns the requirement string" do
+        expect(requirement_string).to eq(">= 3.9, <3.13")
+      end
+    end
+
+    context "without Python version specified" do
+      let(:files) { [] }
+
+      it "returns the highest version" do
+        expect(requirement_string).to eq("3.14.0")
+      end
+    end
+  end
+
+  describe "#python_major_minor" do
+    subject(:major_minor) { manager.python_major_minor }
+
+    context "with requires-python = '>= 3.9, <3.13'" do
+      let(:files) do
+        [
+          Dependabot::DependencyFile.new(
+            name: "pyproject.toml",
+            content: <<~TOML
+              [project]
+              requires-python = ">= 3.9, <3.13"
+            TOML
+          )
+        ]
+      end
+
+      it "returns the major.minor version" do
+        expect(major_minor).to eq("3.9")
+      end
+    end
+  end
+end