BUG: Insert outline items in front of before and cover more of PdfWriter - #4415

…riter

PdfWriter.add_outline_item(before=item) appended the new item at the
end unless `item` was the last one of its level, and inserting in front
of the first item lost the new item: it was counted in /Count, but not
linked into the outline, as /First of the parent was not updated
(ISO 32000-2:2020, 12.3.3, Table 150). Closes #4340.

TreeObject.insert_child() looked for `before` starting at /Last and
following /Next, so it only found the last child. It now starts at
/First when `before` is given and updates /First when inserting in
front of the first child. Appending without `before` still starts at
/Last, so it stays O(1). A `before` that is no child of the parent
still appends at the end.

Add tests for PdfWriter branches without coverage so far:

* add_outline_item() with `before` being None, the first, a middle, the
  last or the only item, without a page (no /A action), and with a
  PageObject,
* the page_mode getter, add_annotation() with an invalid page type and
  clean_page() converting a named destination inside an /A action,
* merge() with excluded_fields=None and with form fields that are not
  on any merged page,
* reset_translation() for a reader the writer never copied from and for
  an IndirectObject, checked by appending the reader again and comparing
  the copied /Resources.

Where the expected behavior comes from ISO 32000-2:2020, the docstring
names the section.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>