…own bubble (#29824)

The compressor has a "double-collision" fallback path: when the
chosen ``summary_role`` collides with the first tail message AND
the flipped role would collide with the last head message, it can't
emit a standalone summary turn (consecutive same-role messages
break Anthropic and friends). It instead prepends the summary +
end-of-summary marker to the first tail message's content via
``_merge_summary_into_tail``.

With the matching anchor from the previous commit, that first tail
message is now usually the user's previously-visible assistant
reply — so the persisted assistant turn ends up shaped as
``[CONTEXT COMPACTION ...] ... --- END OF CONTEXT SUMMARY --- ...
THE ACTUAL REPLY``. Without splitting it, the session viewer
renders one big "Context handoff" bubble and the reply text is
buried inside the metadata blob — which is exactly the
"can't see the last reply" experience #29824 reports, just one
layer deeper.

Added ``splitCompactionContent`` that detects the merge marker
(kept in sync with ``--- END OF CONTEXT SUMMARY — respond to the
message below, not the summary above ---`` in
``agent/context_compressor.py``) and ``MessageBubble`` now
recurses on the two halves: the prefix half renders as the muted
"Context handoff" row, the remainder half renders with the
original assistant styling. Pure (non-merged) summary messages
hit the no-remainder branch and still render as a single
"Context handoff" row, preserving the original behaviour.

…paction rollup (#29824)

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.