fix: Strip backslash before # in generated docstrings

[Alan4506]

Description of changes:
The MarkdownConverter strips unnecessary backslash escapes that pandoc adds for markdown syntax characters. However, # was missing from the strip list. When pandoc converts service documentation containing a literal # character, it escapes it to \# because # is a markdown heading marker. This \# then ends up in the generated Python docstring as an invalid escape sequence, causing pyright reportInvalidStringEscapeSequence errors.

This issue was identified during generation of Lex Runtime V2 client using its model file.

This PR adds # to the regex character class in postProcessPandocOutput() so that \# is correctly stripped to # in generated docstrings.

Verified by generating the Lex Runtime V2 client with the fix. smithy build passes ruff and pyright cleanly, and there is no \# in generated code.

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[arandito]

nit: We should be careful with the characters we unescape. Since our docstrings are in markdown, # is a format specifier for headings if it's at the beginning of a line. If we ever have a case where we unescape and it exists at the beginning of a line, it will lead to the entire line being a heading.

It's definitely a small risk but I think we may be able to escape it as \\# to avoid this issue entirely. However, this is also an issue for any Markdown character that can change the line formatting (e.g.> for code blocks). We might consider a double backslash for all the markdown characters but that makes the in-code docstrings more noisy.

This should be fine for now, but we should follow up to update how we escape markdown characters if it becomes a large issue.

[Alan4506]

It makes sense to me. We should look into it more deeply if there is a large issue in the future.