internal/runtime: fix Markdown conversion on macro return

[gazerro]

internal/runtime: fix Markdown conversion on macro return

Previously Markdown-to-HTML conversion after macro calls wrongly used
the caller format instead of the macro format. This caused Markdown
macro output to be lost in HTML contexts.

This commit aligna macro return conversion with call-time semantics so 
conversion is driven by the called macro format, and add regression
tests for direct and indirect Markdown macro calls from HTML templates.