fix(filter): handle Python single-line docstrings in MinimalFilter

[guyoron1]

Summary

• Fix MinimalFilter state corruption when encountering Python single-line docstrings like """Return the sum."""
• The in_docstring flag was toggled to true but never reset when the closing """ appeared on the same line
• All subsequent lines were treated as docstring content, preventing comment stripping and reducing token savings

Fix

After toggling in_docstring on, check if the rest of the line (after the opening """) contains a closing """:

if !in_docstring {
    in_docstring = true;
    if trimmed.len() > 3 && trimmed[3..].contains("\"\"\"") {
        in_docstring = false;
    }
} else {
    in_docstring = false;
}

Test plan

• Added test_python_single_line_docstring_does_not_corrupt_state test
• Verifies: docstring content kept, subsequent comment stripped, code preserved
• cargo clippy --all-targets passes
• cargo test --all passes

Fixes #2712