Streams where content_blocks carries string index identifiers (e.g.
OpenAI responses/v1 mode with 'lc_rs_305f30', 'lc_txt_1') collapsed
all blocks to wire index 0 because _iter_protocol_blocks fell back to
positional i for non-int indices. Every block appeared as a delta of
block 0, and only one content-block-finish event fired at the end of
the stream.
Keep the raw block key (int or string) internally, allocate sequential
uint wire indices per distinct block, and finish the previously-open
block when a new block key appears — matching the protocol's
no-interleave rule.
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