Fix chat demo role styles so markdown bold is visible

[ryanoneill]

Summary

• Remove BOLD modifier from User and Assistant role styles in the chat markdown demo
• The role style sets the base content style for message text. The header (username) line adds BOLD separately. With BOLD already on the base style, markdown **bold** was visually identical to plain text — no way to tell bold from non-bold.

Test plan

• cargo run --example chat_markdown_demo --features "compound-components,markdown" — type **Hello** how are you doing? and verify "Hello" appears bold while the rest does not

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.71%. Comparing base (5bf5c49) to head (16c4240).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #241   +/-   ##
=======================================
  Coverage   91.71%   91.71%           
=======================================
  Files         148      148           
  Lines       13052    13052           
=======================================
  Hits        11970    11970           
  Misses       1082     1082           

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