feat(matrix): render bot messages with HTML formatting

Add HTML formatted_body to Matrix bot messages so that markdown-style
formatting (code blocks, bold, italic, lists) renders properly in Matrix
clients. Uses the pulldown-cmark crate to convert markdown to HTML and
sets the message format to org.matrix.custom.html.

Story: 188_story_render_matrix_bot_messages_with_html_formatting

Cargo.lock

@@ -2766,6 +2766,24 @@ dependencies = [
  "syn 2.0.116",
 ]
 
+[[package]]
+name = "pulldown-cmark"
+version = "0.12.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f86ba2052aebccc42cbbb3ed234b8b13ce76f75c3551a303cb2bcffcff12bb14"
+dependencies = [
+ "bitflags 2.11.0",
+ "memchr",
+ "pulldown-cmark-escape",
+ "unicase",
+]
+
+[[package]]
+name = "pulldown-cmark-escape"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "007d8adb5ddab6f8e3f491ac63566a7d5002cc7ed73901f72057943fa71ae1ae"
+
 [[package]]
 name = "quick-xml"
 version = "0.36.2"
@@ -3771,6 +3789,7 @@ dependencies = [
  "poem",
  "poem-openapi",
  "portable-pty",
+ "pulldown-cmark",
  "reqwest 0.13.2",
  "rust-embed",
  "serde",