fix(normalization): preserve blank-line runs and trailing spaces inside fenced code blocks (#2528)

[İlker Oğulcan SAYIN (iosayin)]

Closes #2528

Summary

During final document normalization in MarkItDown, the text processing pipeline previously applied a blanket line.rstrip() and collapsed 3+ newline runs (re.sub(r"\n{3,}", "\n\n", ...)) uniformly across the entire document without distinguishing between prose and code fences. This resulted in data loss inside fenced code blocks (````` or ~~~), collapsing consecutive blank lines and stripping semantically significant trailing spaces.

Solution

• Make final markdown normalization fence-aware: text outside code fences undergoes standard line-trimming and newline collapse, while content inside fenced code blocks preserves exact trailing whitespace and consecutive blank lines.
• Added regression unit tests in tests/test_module_misc.py.