Fix compile-time link validation and add tag page links

- Restore HTML link extraction in LinkValidator (removed in a83634d
  under the false premise that post bodies are raw markdown; they
  are HTML rendered by NimblePublisher at compile time). The missing
  regex made extract_links/1 find zero links, silently disabling
  compile-time validation.
- Support /blog/{blog_id}/tag/{tag} links: validate blog ID,
  require non-empty tag (tags are user-defined, e.g. pi.dev).
- Fix invalid links in two posts: tag/Pi.dev -> tag/pi.dev,
  2026-07-13-synthetic-tdd.md -> synthetic-tdd.
- Fix test warnings: use Plug.Test deprecation, unused import,
  runtime-defined TestBlogValid module.
- Add regression tests for HTML extraction and tag page links.

blogex/lib/blogex/link_validator.ex

@@ -21,6 +21,12 @@ defmodule Blogex.LinkValidator do
     * Must not contain consecutive hyphens
     * Query strings and anchor fragments are allowed after the slug
 
+  ## Tag page links
+
+    * `/blog/{blog_id}/tag/{tag}` links point to tag pages
+    * The tag may not be empty; otherwise any non-empty tag is accepted
+      (tags are user-defined and may contain dots, uppercase letters, etc.)
+
   ## Usage
 
       # Validate a single link
@@ -50,21 +56,34 @@ defmodule Blogex.LinkValidator do
   `/blog/{engineering|releases}/{slug}`. External links and non-blog
   internal links are ignored.
 
-  Handles markdown link syntax `[text](url)`.
+  Handles both markdown link syntax `[text](url)` and HTML `<a href="url">`.
+  (Post bodies are HTML, rendered by NimblePublisher at compile time.)
 
   ## Examples
 
       iex> extract_links("[link](/blog/engineering/post)")
       ["/blog/engineering/post"]
 
+      iex> extract_links("<p><a href=\"/blog/engineering/post\">link</a></p>")
+      ["/blog/engineering/post"]
+
       iex> extract_links("See [GitHub](https://github.com)")
       []
   """
   @spec extract_links(String.t()) :: [String.t()]
   def extract_links(body) when is_binary(body) do
-    ~r/\[([^\]]+)\]\(([^)]+)\)/
-    |> Regex.scan(body)
-    |> Enum.map(fn [_, _, path] -> path end)
+    markdown_links =
+      ~r/\[([^\]]+)\]\(([^)]+)\)/
+      |> Regex.scan(body)
+      |> Enum.map(fn [_, _, path] -> path end)
+
+    html_links =
+      ~r/<a\s+href=["']([^"']*)["']/i
+      |> Regex.scan(body)
+      |> Enum.map(fn [_, path] -> path end)
+
+    (markdown_links ++ html_links)
+    |> Enum.uniq()
     |> Enum.filter(&internal_blog_link?/1)
   end
 
@@ -90,6 +109,9 @@ defmodule Blogex.LinkValidator do
 
       iex> validate_link("/blog/engineering/My-Post")
       {:error, "slug must be lowercase alphanumeric with hyphens: My-Post"}
+
+      iex> validate_link("/blog/engineering/tag/pi.dev")
+      :ok
   """
   @spec validate_link(String.t()) :: :ok | {:error, String.t()}
   def validate_link(link) when is_binary(link) do
@@ -99,7 +121,13 @@ defmodule Blogex.LinkValidator do
 
       {blog_id_str, slug_part} ->
         case Map.fetch(@valid_blog_ids, blog_id_str) do
-          {:ok, _blog_atom} -> validate_slug(slug_part)
+          {:ok, _blog_atom} ->
+            if String.starts_with?(slug_part, "tag/") do
+              validate_tag(String.replace_prefix(slug_part, "tag/", ""))
+            else
+              validate_slug(slug_part)
+            end
+
           :error -> {:error, "unknown blog ID: #{blog_id_str}"}
         end
     end
@@ -209,4 +237,12 @@ defmodule Blogex.LinkValidator do
       {:error, "slug must be lowercase alphanumeric with hyphens: #{slug}"}
     end
   end
+
+  @doc false
+  @spec validate_tag(String.t()) :: :ok | {:error, String.t()}
+  defp validate_tag(tag) when tag == "" do
+    {:error, "empty tag in tag link"}
+  end
+
+  defp validate_tag(_tag), do: :ok
 end