`PaginationInfo.__post_init__()` rejects a negative `total_items`, which
is right for ordinary construction but leaves the wire with nowhere to
go: a message breaking the invariant can only raise. Every other wrapper
with a whole-object rule already has a valid/invalid pair, so give
pagination one too.

`BasePaginationInfo` holds the two shared fields and refuses direct
instantiation, `PaginationInfo` keeps the `ValueError` unchanged, and
`InvalidPaginationInfo` accepts the same fields with no invariants so a
malformed count survives for inspection. There is deliberately no escape
hatch on `PaginationInfo`: unlike `DeliveryArea`, it has always been
strict here, so there is no released behavior to transition away from.

Both leaves gain a `__str__`, which neither had: the pair only reads
well if the invalid marker composes with a valid form, and the dataclass
`repr` is too long for a log line. The rule is per-field, so the marker
goes on the count (`items=<invalid:-1>,next=None`) rather than around
the whole object, as in `InvalidDeliveryArea`.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

`pagination_info_from_proto()` calls the `PaginationInfo` constructor
directly, so it raises on a count the constructor rejects. A conversion
function must never raise on wire content: the caller loses the whole
message over one bad field. Add the replacement that returns
`PaginationInfo | InvalidPaginationInfo` and deprecate the old one,
following the path `delivery_area_from_proto` and `bounds_from_proto`
already took.

The warning text uses the form the deprecation guide prescribes, with
both fully qualified names written out. The six deprecations already in
the library use a shorter backticked form instead, so this one does not
match them; the guide is the normative source, and a caller who sees
only the warning can act on it without guessing the import path.

The new converter also reads `next_page_token` through `HasField()`
instead of testing it for truth. The field has presence, so the old
truthiness test collapses "the server sent an empty token" into "the
server sent no token"; only the second means there are no more pages.
Changing this in place would be a silent behavior change, which is
another reason it belongs in the new name rather than the old one.

Note that `total_items` is a `uint32`, so a decoded message cannot in
fact carry a negative count today, and the invalid branch is
unreachable through the generated bindings. It is there because the
converter's contract is not to raise regardless of what it is handed,
and because the wire type is not the wrapper's to assume; the test
exercises it with a message-like stand-in.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>