fix(cli): --zero breaks open-ended column ranges

[VXNCXNX]

What's broken

--zero breaks open-ended column ranges.

$ printf 'a,b,c\n1,2,3\n' > abc.csv

$ csvcut -c 1: abc.csv
a,b,c

$ csvcut --zero -c 0: abc.csv
ColumnIdentifierError: Column 3 is invalid. The last column is 'c' at index 2.

Same intent, expressed in the offset the flag asks for, and it errors. The other end is wrong too, silently:

$ csvcut --zero -c :1 abc.csv
b                      <- should be a,b

-C/--not-columns shares the code and fails the same way:

$ csvcut --zero -C 1: abc.csv
ColumnIdentifierError: Column 3 is invalid...

The fix

In _resolve_column_identifier, the implicit ends of an open-ended range were hardcoded for 1-based identifiers:

a = int(a) if a else 1
b = int(b) + 1 if b else len(column_names) + 1

Both bounds live in identifier space, so both depend on column_offset. Under --zero the upper bound overshot by one and tripped the validity guard, and the lower bound started at the second column.

All six shapes after the change:

-c 1:          a,b,c        --zero -c 0:    a,b,c
-c :2          a,b          --zero -c :1    a,b
-c 2:3         b,c          --zero -c 1:2   b,c
-C 2:          a            --zero -C 1:    a

Not undoing the recent exclusion fix

The Unreleased changelog has fix: -C/--not-columns now excludes the last column of an open-ended range (e.g. 2-), which is what the + 1 on the upper bound is for. Under the default column_offset=1, len + column_offset is exactly the old len + 1, so that behaviour is untouched and only the column_offset=0 case changes. Its guard tests, test_exclude_open_ended_range and test_exclude_ignores_unknown_names_but_reports_invalid_ranges, still pass, and -C 2: still yields a.

Verification

test_range_notation_open_ended tested only column_offset=1, while its sibling test_range_notation covers both offsets. The missing offset-0 rows are added, for :3, 1,4:, 0:, and both offsets of the open-ended exclusion form.

Reverting either bound alone fails the test, so both halves are independently covered.

Suite unchanged: 351 passed under en_US.UTF-8 before and after. Under de_DE.UTF-8 it is 4 failed before and after, all pre-existing and in other utilities.

Changelog entry added under Unreleased.