test: migrate tests from centralized mjs to colocated TypeScript

Move test files from tests/ directory to colocate with source code,
convert from .mjs to .ts using tsx runner, add workspaces and npm
cache to CI workflow.

packages/baoyu-md/src/content.test.ts

@@ -0,0 +1,93 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import {
+  extractSummaryFromBody,
+  extractTitleFromMarkdown,
+  parseFrontmatter,
+  pickFirstString,
+  serializeFrontmatter,
+  stripWrappingQuotes,
+  toFrontmatterString,
+} from "./content.ts";
+
+test("parseFrontmatter extracts YAML fields and strips wrapping quotes", () => {
+  const input = `---
+title: "Hello World"
+author: ‘Baoyu’
+summary: plain text
+---
+# Heading
+
+Body`;
+
+  const result = parseFrontmatter(input);
+
+  assert.deepEqual(result.frontmatter, {
+    title: "Hello World",
+    author: "Baoyu",
+    summary: "plain text",
+  });
+  assert.match(result.body, /^# Heading/);
+});
+
+test("parseFrontmatter returns original content when no frontmatter exists", () => {
+  const input = "# No frontmatter";
+  assert.deepEqual(parseFrontmatter(input), {
+    frontmatter: {},
+    body: input,
+  });
+});
+
+test("serializeFrontmatter renders YAML only when fields exist", () => {
+  assert.equal(serializeFrontmatter({}), "");
+  assert.equal(
+    serializeFrontmatter({ title: "Hello", author: "Baoyu" }),
+    "---\ntitle: Hello\nauthor: Baoyu\n---\n",
+  );
+});
+
+test("quote and frontmatter string helpers normalize mixed scalar values", () => {
+  assert.equal(stripWrappingQuotes(`" quoted "`), "quoted");
+  assert.equal(stripWrappingQuotes("“ 中文标题 ”"), "中文标题");
+  assert.equal(stripWrappingQuotes("plain"), "plain");
+
+  assert.equal(toFrontmatterString("'hello'"), "hello");
+  assert.equal(toFrontmatterString(42), "42");
+  assert.equal(toFrontmatterString(false), "false");
+  assert.equal(toFrontmatterString({}), undefined);
+
+  assert.equal(
+    pickFirstString({ summary: 123, title: "" }, ["title", "summary"]),
+    "123",
+  );
+});
+
+test("markdown title and summary extraction skip non-body content and clean formatting", () => {
+  const markdown = `
+![cover](cover.png)
+## “My Title”
+
+Body paragraph
+`;
+  assert.equal(extractTitleFromMarkdown(markdown), "My Title");
+
+  const summary = extractSummaryFromBody(
+    `
+# Heading
+> quote
+- list
+1. ordered
+\`\`\`
+code
+\`\`\`
+This is **the first paragraph** with [a link](https://example.com) and \`inline code\` that should be summarized cleanly.
+`,
+    70,
+  );
+
+  assert.equal(
+    summary,
+    "This is the first paragraph with a link and inline code that should...",
+  );
+});