Trailing backslashes break name round trips

[claell]

Describe the bug

Trailing backslashes break the intended inverse relationship between
NameParts.merge_last_name_first and parse_single_name_into_parts.

Two related cases are involved:

• Parsing a name whose final token ends in a backslash appends that backslash
  twice because the end-of-input branch does not stop further handling of the
  same character.
• merge_last_name_first escapes the final section even though no comma follows
  it, changing a terminal backslash unnecessarily.

The existing inverse-property test skips strict and non-strict examples with
odd trailing backslashes, concealing the round-trip failure.

Reproducing

Version: current main (08e370a)

Code:

from bibtexparser.middlewares.names import NameParts
from bibtexparser.middlewares.names import parse_single_name_into_parts

parts = NameParts(
    first=["Christina", "A.", "L.\\"],
    von=[],
    last=["Thiele"],
    jr=[],
)
merged = parts.merge_last_name_first
assert parse_single_name_into_parts(merged) == parts

The assertion fails on current main.

Expected behavior

Last-name-first serialization and reparsing should preserve the NameParts
value in both strict and non-strict modes, including terminal backslashes. Only
sections followed by a comma need delimiter-protecting escaping.

Workaround

Avoid terminal backslashes in name parts or normalize affected names manually.

Remaining Questions (Optional)

• I would be willing to contribute a PR to fix this issue.
• This issue is a blocker, I'd be grateful for an early fix.

[claell]

Draft PR #573 contains the focused parser/serializer fix and enables the previously skipped strict and non-strict inverse-property cases.