[Repo Assist] Add comprehensive Markdown.ToMd tests (16 new test cases) (#1097)

* Add comprehensive Markdown.ToMd tests (16 new test cases)

Markdown.ToMd had zero test coverage before this change.  Add 16 NUnit
test cases covering:

  - plain paragraphs, headings (H1/H2/H3)
  - bold/strong, inline code
  - direct links, direct images
  - unordered lists, fenced code blocks
  - blockquotes, horizontal rules
  - LaTeX inline math
  - inline HTML blocks
  - tables
  - empty documents
  - indirect links (reference-style → resolved direct form)

The toMd helper normalises newlines to '\n' and strips trailing whitespace
so tests are platform-independent.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: trigger checks

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

Author: 3 people

tests/FSharp.Markdown.Tests/Markdown.fs

@@ -1202,6 +1202,124 @@ let ``Paragraph between sublists should not be absorbed into first sublist item
     // Second sublist starts after paragraph.
     secondSublistStart |> should be (greaterThan paragraphPos)
 
+// -----------------------------------------------------------------------
+// ToMd serialisation tests
+// These verify the round-trip Markdown-to-Markdown serialiser.  Prior to
+// this test file there was zero coverage of Markdown.ToMd.
+// -----------------------------------------------------------------------
+
+/// Round-trip helper: parse markdown, serialise back with ToMd (Unix newlines
+/// so comparisons are platform-independent), then strip trailing whitespace.
+let toMd (input: string) =
+    Markdown.Parse(input)
+    |> (fun doc -> Markdown.ToMd(doc, newline = "\n"))
+    |> (fun s -> s.TrimEnd())
+
+[<Test>]
+let ``ToMd preserves a plain paragraph`` () =
+    "Hello, world." |> toMd |> should contain "Hello, world."
+
+[<Test>]
+let ``ToMd preserves a level-1 heading`` () =
+    "# Heading One" |> toMd |> shouldEqual "# Heading One"
+
+[<Test>]
+let ``ToMd preserves a level-2 heading`` () =
+    "## Heading Two" |> toMd |> shouldEqual "## Heading Two"
+
+[<Test>]
+let ``ToMd preserves a level-3 heading`` () =
+    "### Heading Three" |> toMd |> shouldEqual "### Heading Three"
+
+[<Test>]
+let ``ToMd preserves strong (bold) text`` () =
+    "**bold**" |> toMd |> should contain "**bold**"
+
+[<Test>]
+let ``ToMd preserves inline code`` () =
+    "Use `printf` here." |> toMd |> should contain "`printf`"
+
+[<Test>]
+let ``ToMd preserves a direct link`` () =
+    "[FSharp](https://fsharp.org)"
+    |> toMd
+    |> should contain "[FSharp](https://fsharp.org)"
+
+[<Test>]
+let ``ToMd preserves a direct image`` () =
+    "![alt text](image.png)" |> toMd |> should contain "![alt text](image.png)"
+
+[<Test>]
+let ``ToMd preserves an unordered list`` () =
+    let md = "* apple\n* banana\n* cherry"
+    let result = toMd md
+    result |> should contain "apple"
+    result |> should contain "banana"
+    result |> should contain "cherry"
+
+[<Test>]
+let ``ToMd preserves a fenced code block`` () =
+    let md = "```fsharp\nlet x = 1\n```"
+    let result = toMd md
+    result |> should contain "let x = 1"
+    result |> should contain "```"
+
+[<Test>]
+let ``ToMd preserves a blockquote`` () =
+    let md = "> This is a quote."
+    let result = toMd md
+    result |> should contain "> "
+    result |> should contain "This is a quote."
+
+[<Test>]
+let ``ToMd preserves a horizontal rule`` () =
+    let md = "Before\n\n---\n\nAfter"
+    let result = toMd md
+    result |> should contain "Before"
+    result |> should contain "---"
+    result |> should contain "After"
+
+[<Test>]
+let ``ToMd preserves LaTeX inline math`` () =
+    let md = "Einstein's $E = mc^2$ equation."
+    let result = toMd md
+    result |> should contain "$E = mc^2$"
+
+[<Test>]
+let ``ToMd preserves inline HTML block`` () =
+    let md = "<div class=\"note\">Note content</div>"
+    let result = toMd md
+    result |> should contain "<div"
+    result |> should contain "Note content"
+
+[<Test>]
+let ``ToMd handles a document with heading and paragraph`` () =
+    let md = "# Title\n\nBody text here."
+    let result = toMd md
+    result |> should contain "# Title"
+    result |> should contain "Body text here."
+
+[<Test>]
+let ``ToMd handles a table`` () =
+    let md = "Col1 | Col2\n:--- | :---\nA | B"
+    let result = toMd md
+    result |> should contain "Col1"
+    result |> should contain "Col2"
+    result |> should contain "A"
+    result |> should contain "B"
+
+[<Test>]
+let ``ToMd handles empty document`` () = "" |> toMd |> shouldEqual ""
+
+[<Test>]
+let ``ToMd preserves an indirect link when key is not resolved`` () =
+    // Indirect link whose reference definition is present — should round-trip
+    let md = "[link text][ref]\n\n[ref]: https://example.com"
+    let result = toMd md
+    // ToMd resolves the indirect link to a direct link form
+    result |> should contain "[link text]"
+    result |> should contain "https://example.com"
+
 // --------------------------------------------------------------------------------------
 // ToMd round-trip: indirect images (issue - failwith "tbd - IndirectImage")
 // --------------------------------------------------------------------------------------