Add support for .tools-versions (#531)

Closes: #504

__tests__/fixtures/.tool-versions

@@ -0,0 +1 @@
+uv 0.5.15

__tests__/version/tool-versions-file.test.ts

@@ -0,0 +1,115 @@
+jest.mock("node:fs");
+jest.mock("@actions/core", () => ({
+  warning: jest.fn(),
+}));
+
+import fs from "node:fs";
+import * as core from "@actions/core";
+import { beforeEach, describe, expect, it, jest } from "@jest/globals";
+import { getUvVersionFromToolVersions } from "../../src/version/tool-versions-file";
+
+const mockedFs = fs as jest.Mocked<typeof fs>;
+const mockedCore = core as jest.Mocked<typeof core>;
+
+describe("getUvVersionFromToolVersions", () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it("should return undefined for non-.tool-versions files", () => {
+    const result = getUvVersionFromToolVersions("package.json");
+    expect(result).toBeUndefined();
+    expect(mockedFs.readFileSync).not.toHaveBeenCalled();
+  });
+
+  it("should return version for valid uv entry", () => {
+    const fileContent = "python 3.11.0\nuv 0.1.0\nnodejs 18.0.0";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBe("0.1.0");
+    expect(mockedFs.readFileSync).toHaveBeenCalledWith(
+      ".tool-versions",
+      "utf8",
+    );
+  });
+
+  it("should return version for uv entry with v prefix", () => {
+    const fileContent = "uv v0.2.0";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBe("0.2.0");
+  });
+
+  it("should handle whitespace around uv entry", () => {
+    const fileContent = "  uv   0.3.0  ";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBe("0.3.0");
+  });
+
+  it("should skip commented lines", () => {
+    const fileContent = "# uv 0.1.0\npython 3.11.0\nuv 0.2.0";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBe("0.2.0");
+  });
+
+  it("should return first matching uv version", () => {
+    const fileContent = "uv 0.1.0\npython 3.11.0\nuv 0.2.0";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBe("0.1.0");
+  });
+
+  it("should return undefined when no uv entry found", () => {
+    const fileContent = "python 3.11.0\nnodejs 18.0.0";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBeUndefined();
+  });
+
+  it("should return undefined for empty file", () => {
+    mockedFs.readFileSync.mockReturnValue("");
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBeUndefined();
+  });
+
+  it("should warn and return undefined for ref syntax", () => {
+    const fileContent = "uv ref:main";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions(".tool-versions");
+
+    expect(result).toBeUndefined();
+    expect(mockedCore.warning).toHaveBeenCalledWith(
+      "The ref syntax of .tool-versions is not supported. Please use a released version instead.",
+    );
+  });
+
+  it("should handle file path with .tool-versions extension", () => {
+    const fileContent = "uv 0.1.0";
+    mockedFs.readFileSync.mockReturnValue(fileContent);
+
+    const result = getUvVersionFromToolVersions("path/to/.tool-versions");
+
+    expect(result).toBe("0.1.0");
+    expect(mockedFs.readFileSync).toHaveBeenCalledWith(
+      "path/to/.tool-versions",
+      "utf8",
+    );
+  });
+});