Fix MDEx being a hard compile-time dependency. v4.0.0 called
MDEx.Document.default_sanitize_options/0 in a module attribute (evaluated at
compile time), so petal_components failed to compile unless the consumer also
added :mdex — defeating the optional dep. Build the markdown options at call
time instead, behind the existing runtime guard.

Verified by compiling the library with no :mdex present (the MCP extractor
project): module now compiles and schema extraction succeeds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>