Keep Unpacker.tell() at the start of an incomplete object - #772

Unpacker.tell() is documented as "the starting position of the next object". With the C extension that stops being true once the buffer ends inside an object. The C unpacker decodes the bytes it has, keeps the partial object for the next feed(), and counts those bytes in stream_offset. So tell() points into the middle of the next object.

This breaks the usual way to resume a stream: read what is there, iterate, save tell(), and continue from that offset later. A minimal case is packb([1, 2, 3]) + packb(["x" * 40, "y" * 40]), a 4-byte object followed by an 85-byte one, with only the first 49 bytes available:

unpacker = msgpack.Unpacker(io.BytesIO(data[:49]), read_size=16)
list(unpacker)    # [[1, 2, 3]]
unpacker.tell()   # 49 with the C extension, 4 with the pure Python one

Starting a new Unpacker at offset 49 decodes forty 121 integers from the middle of "y" * 40, with no error. The same happens with feed(). The pure Python implementation rolls back to the start of the incomplete object, so it returns 4.

The C unpacker now records stream_offset when it starts a new object, and tell() returns that value while ctx.execute shows an incomplete object. Nothing changes once the object is complete or after an error, and read_bytes() already refuses to run while an object is incomplete. The pure Python implementation is unchanged.

test_unpack_tell_incomplete_object in test/test_sequnpack.py cuts the stream at three points inside the second object, for both feed() and a file-like object. It checks that tell() is 4 after the first object and 89 once the rest has arrived. With only the tell() change reverted, it fails on the C extension with assert 5 == 4, and it passes on both implementations with the change. The full suite passes with the C extension (179 passed, 1 skipped) and with MSGPACK_PUREPYTHON=1 (176 passed, 4 skipped), and ruff check and ruff format report no changes. I ran this on CPython 3.13 on Linux only. I did not run the free-threaded build or the parallel-thread tests.