21 cases pinning the new ``_ensure_last_assistant_message_in_tail``
anchor and its interaction with the existing tail-cut path:
* ``TestFindLastAssistantMessageIdx`` — helper contract: prefers a
content-bearing assistant message, skips ``tool_calls``-only
stubs, multimodal text-block content counts, falls back to
"any assistant" when no content-bearing reply exists, honours
``head_end``, returns -1 when there's none.
* ``TestEnsureLastAssistantMessageInTail`` — direct: no-op when
already in the tail, walks ``cut_idx`` back when the reply is
in the compressed middle, never crosses into the head region,
re-aligns through a preceding ``tool_call`` / ``tool_result``
group instead of orphaning it.
* ``TestFindTailCutByTokensAnchorsAssistant`` — integration:
reporter repro (long tool-output run after the visible reply)
now preserves the reply; user and assistant anchors compose
in a single tail-cut call; a soft-ceiling-overrunning oversized
tool result no longer strands the prior reply.
* ``TestCompactionRollupReproduction`` — end-to-end through
``compress()`` with a stubbed ``_generate_summary``: the
visible reply text survives either as its own standalone
assistant message (normal path) or concatenated onto the
merged summary tail (double-collision path the WebUI then
re-splits). The standalone-summary case is asserted strictly
(exactly one summary row, exactly one separate assistant
row carrying the reply) — that's the dominant path and any
drift there reintroduces the original bug.
* ``TestSourceGuardrail`` — static asserts on
``agent/context_compressor.py``: the helper exists, the
anchor is wired into ``_find_tail_cut_by_tokens`` AFTER the
user-message anchor (so chaining is monotonic), the
content-bearing preference is preserved, and the issue
number is referenced so future bisects can find this fix.
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