fenced code blocks lose blank-line runs and trailing spaces during conversion

what happens

converting markdown through MarkItDown corrupts fenced code blocks:

• two or more consecutive blank lines inside a fence collapse to one
• trailing spaces on lines inside a fence get stripped

def a():
    pass


def b():
    pass

comes back with the double blank line gone, and line with trailing spaces loses its spaces.

why

the result normalization (rstrip each line, collapse 3+ newline runs) runs over the whole document with no awareness of code fences, so it mangles content where whitespace is significant.

expected

whitespace inside fenced code blocks (``` or ~~~) survives exactly; normalization still applies to prose outside fences.

env: main 945314a

Locate the result-normalization path in the Markdown conversion entry point and first reproduce the issue with the fenced examples from this report. Add regression coverage for both ``` and ~~~ fences, including repeated blank lines and trailing spaces, and verify that normalization still affects prose outside fences.