fix: dedent indented code blocks before formatting

[mokevnin]

Fixes #1702.

Problem

A code block nested in a list item keeps its list indentation when it is handed to the formatter, and indent_codeblock puts the same indentation back afterwards. Every line except the first is indented twice, so the file never converges — mdsf verify reports changes however many times mdsf format has run.

1. Step

    ```python
    a = 1
    b = 2
    ```

After one mdsf format the block reads a = 1 at 4 spaces and b = 2 at 8.

Formatters that normalize indentation themselves (oxfmt, shfmt) mask it — they receive the broken snippet and straighten it out. Ones that do not, reject it: ruff reads the extra indentation as IndentationError, so the block is silently left unformatted.

Change

• strip the fence indentation from every snippet line before it reaches the formatter (dedent_line);
• stop adding indentation to empty lines — they used to come back as trailing whitespace.

Checks

cargo test -p mdsf --lib passes, including four new tests for parse_generic_codeblock and indent_codeblock. custom::test_custom_tool::with_stdin_true fails on my machine both with and without this change (missing tool binary), so it is unrelated.

Measured on a corpus of 247 markdown files that contain indented code blocks. With 0.12.1, one mdsf format pass followed by mdsf verify left 108 of them still reporting changes; with this change, 17 do. Those 17 all have a fence nested inside another fence, which is a separate limitation — the inner closing fence ends the outer block for the line scanner.