bug: need to fill in the MarkdownStyleSheet decorations

[csells]

The LlmMessageStyle has embedded a MarkdownStyleSheet. It has values for the important things except these four decorations:

• blockquoteDecoration
• codeblockDecoration
• horizontalRuleDecoration
• tableCellsDecoration

These should be set in the lightStyles method and inverted in the darkStyles method like so:

        blockquoteDecoration:
            sh.invertDecoration(markdownStyle.blockquoteDecoration),
        codeblockDecoration:
            sh.invertDecoration(markdownStyle.codeblockDecoration),
        horizontalRuleDecoration:
            sh.invertDecoration(markdownStyle.horizontalRuleDecoration),
        tableCellsDecoration:
            sh.invertDecoration(markdownStyle.tableCellsDecoration),

Otherwise, when these decorations are used, they'll be whatever default they are and not a) match the styles of the LlmChatView and b) won't switch to dark.