gh-158398: Clear unconsumed_tail at end of stream

[Tony123-tech]

Fixes #158398.

The test test_decompress_unconsumed_tail_after_eof (added in gh-158169 / #158177) fails on main:

AssertionError: b'0123456789' != b''

Root cause: gh-158169 added self->zst.next_in += left_size; to save_unconsumed_input(), which advances next_in before the following block computes the remaining input length. This makes the computation wrong, so unconsumed_tail is not cleared at end of stream.

Changes:

• Revert the self->zst.next_in += left_size; line.
• Clear unconsumed_tail explicitly when Z_STREAM_END is reached.
• Make the trailing block an else if so it is not entered at end of stream.

• Issue: zlib.Decompress.unconsumed_tail not cleared at end of stream (regression from gh-158169) #158398