Fix empty text after trimming whitespace

[lntutor]

Config::trim_text_end promises not to emit a text event when trimming leaves it empty, but whitespace-only text immediately before markup still produced Text("").

This change lets both slice and buffered readers skip those whitespace-only spans before constructing an event. read_to_end temporarily disables both start and end trimming so its byte-range accounting remains unchanged.

Regression coverage exercises both Reader::from_str and buffered Reader::from_reader.

Verification:

• cargo fmt --all -- --check
• cargo test skips_whitespace_only_text_before_markup --all-features
• cargo test --all-features
• git diff --check

Closes #984

[lntutor]

Closing as a duplicate of #986, which was already open for the same reported behavior. Sorry for the overlap.