Text-batching tests (and any tooling) build MatrixAdapter via
object.__new__ without running __init__; moving _split_threshold from a
class constant to an instance attribute made _flush_text_batch die with
AttributeError, silently dropping the flush. Restore class-level
defaults (max_message_length, _split_threshold) that __init__ overrides,
and derive the near-limit test payload from adapter._split_threshold
instead of the old hardcoded 3950.
0e281b58e6