fix(filter): preserve code around block comments

[lntutor]

Summary

• remove block-comment spans instead of dropping every line containing a comment marker
• preserve code before and after same-line comments
• preserve code before an opening comment and after a later closing delimiter
• support multiple block-comment spans per line while retaining Rust doc blocks
• trim only horizontal whitespace left at the end of stripped lines

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features (2,498 passed, 8 ignored)

Closes #2714

[CLAassistant]

All committers have signed the CLA.