test: add unit and integration test suite

Add 34 tests (27 unit, 7 integration) using node:test runner:

Unit tests:
- pickServer(), findRoot(), pathToUri(), uriToPath()
- isLspCommand(), listCommands()
- formatHover(), formatDefinition(), formatReferences(), formatDiagnostics()

Integration tests:
- daemon lifecycle (status/stop) on isolated socket
- CLI --no-daemon queries (hover, documentSymbol, diagnostics)

Supporting changes:
- socketPath() honors PI_LSP_SOCKET_PATH env var for test isolation
- test fixtures for valid and broken TypeScript files
- npm test / test:unit / test:integration scripts

test/unit/root.test.ts

@@ -0,0 +1,87 @@
+// Root Unit Tests — pickServer(), findRoot(), pathToUri(), uriToPath().
+import { describe, it } from "node:test";
+import * as assert from "node:assert/strict";
+import * as path from "node:path";
+import { fileURLToPath } from "node:url";
+import { pickServer, findRoot, pathToUri, uriToPath } from "../../src/root.ts";
+import { UnsupportedExtensionError } from "../../src/types.ts";
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const projectRoot = path.resolve(__dirname, "..", "..");
+
+describe("pathToUri / uriToPath", () => {
+  it("converts a relative path to a file URI", () => {
+    const uri = pathToUri("cli.ts");
+    assert.ok(uri.startsWith("file://"));
+    assert.ok(uri.endsWith("cli.ts"));
+  });
+
+  it("converts an absolute path to a file URI", () => {
+    const absPath = path.resolve(projectRoot, "cli.ts");
+    const uri = pathToUri(absPath);
+    assert.strictEqual(uri, `file://${absPath}`);
+  });
+
+  it("round-trips path -> uri -> path", () => {
+    const absPath = path.resolve(projectRoot, "src", "client.ts");
+    const uri = pathToUri(absPath);
+    const back = uriToPath(uri);
+    assert.strictEqual(back, absPath);
+  });
+});
+
+describe("pickServer", () => {
+  it("picks gopls for .go files", () => {
+    const server = pickServer("/some/path/file.go");
+    assert.strictEqual(server.id, "gopls");
+  });
+
+  it("picks typescript-language-server for .ts files", () => {
+    const server = pickServer("/some/path/file.ts");
+    assert.strictEqual(server.id, "typescript-language-server");
+  });
+
+  it("picks typescript-language-server for .tsx files", () => {
+    const server = pickServer("/some/path/file.tsx");
+    assert.strictEqual(server.id, "typescript-language-server");
+  });
+
+  it("picks pyright for .py files", () => {
+    const server = pickServer("/some/path/file.py");
+    assert.strictEqual(server.id, "pyright");
+  });
+
+  it("throws UnsupportedExtensionError for unknown extensions", () => {
+    assert.throws(
+      () => pickServer("/some/path/file.xyz"),
+      UnsupportedExtensionError,
+    );
+  });
+});
+
+describe("findRoot", () => {
+  it("finds project root using go.mod marker", () => {
+    // The project root has package.json but not go.mod, so use a file in the
+    // project and look for tsconfig.json.
+    const root = findRoot(
+      path.join(projectRoot, "cli.ts"),
+      ["tsconfig.json"],
+    );
+    assert.strictEqual(root, projectRoot);
+  });
+
+  it("finds nearest directory containing marker", () => {
+    // The test/fixtures/ dir doesn't have tsconfig.json, but the project root does.
+    const fixtureFile = path.join(projectRoot, "test", "fixtures", "sample.ts");
+    const root = findRoot(fixtureFile, ["tsconfig.json"]);
+    assert.strictEqual(root, projectRoot);
+  });
+
+  it("falls back to file's directory when no marker found", () => {
+    const root = findRoot(
+      path.join(projectRoot, "cli.ts"),
+      ["nonexistent-marker-xyz"],
+    );
+    assert.strictEqual(root, projectRoot);
+  });
+});