Flush Python stream item markers before unchecked writes

[KairosOps]

Fixes #289.

The per-item streaming path in the Python binary runtime writes the stream item marker with write_byte_no_check(1). When an element lands exactly at the end of the output buffer, the next marker writes past the bytearray before any element serializer gets a chance to flush.

This gives the marker the same one-byte capacity guarantee used by the nearby tag writers. I added a small regression test that uses a 2-byte buffer and optional values so the boundary is hit deterministically.

Checked locally:

• python -m pytest python\tests\test_binary_stream_writer.py -q
• python -m py_compile tooling\internal\python\static_files\_binary.py python\tests\test_binary_stream_writer.py
• git diff --cached --check