Restore a placeholder a stream-mode reply plugin splits in two

[fylorn]

Problem

In stream mode (reply: "stream") the gateway restored placeholders in each string a reply plugin handed back on its own (Chain::stream_call and the onReplyTextEnd output, which whole bodies also go through). A plugin that holds back the end of its text can cut a <<TW_SECRET_n>> placeholder in two. Neither half matches the ledger, so the client received the placeholder text instead of the original value, and the next plugin in the chain saw the two unrestored halves.

Change

• Bridge::unfinished_from: from which byte a string ends in the unfinished start of a placeholder in the ledger (binary search over the placeholders, same approach as hold_from for values). Only ledger placeholders count, so a << in code waits at most for the plugin's next piece.
• Per plugin and per text block, a trailing unfinished placeholder in the plugin's output is held and restored together with the next piece it hands back. At the end of the block it is joined with the onReplyTextEnd output; an incomplete leftover goes out as is. On the skip-on-error paths the held piece is given back with the rest.
• Same path: a stream-mode plugin skipped after failing in onReplyTextEnd lost the text it was holding, because the block's state was removed before the call. The state is now kept until the block ends.
• Same path: when several text blocks are open at once and a plugin is skipped after failing in one of them, what it was still holding in the other blocks came out only when those blocks ended, after their later pieces. It now goes out in front of the next piece of its block.

Tests

New tests in plugin/reply/tests use a stream plugin that hands back all but the last few characters and releases them in onReplyTextEnd, with a secret from the request in the answer. All of them failed before the change (the client got use <<TW_SECRET_1>> now; the end-of-block skip case got an empty block):

• a_placeholder_a_stream_plugin_splits_is_restored_from_both_halves (several delta splits, one and two plugins, two chunk sizes)
• the_next_plugin_in_the_chain_sees_whole_placeholders
• text_given_back_when_a_plugin_is_skipped_comes_out_restored
• a_stream_plugin_failing_at_the_end_of_a_block_gives_back_what_it_held
• whole_bodies_restore_a_placeholder_a_stream_plugin_splits (all four formats)
• a_plugin_skipped_in_one_block_gives_back_another_open_blocks_text_in_order (two interleaved Anthropic text blocks; before the change the second block came out reordered)

Plus a unit test for Bridge::unfinished_from. cargo fmt --check, cargo clippy --workspace --all-targets -D warnings and cargo test --workspace pass locally.

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