…tes the latest turn

Extended-thinking Claude models (4.6+, e.g. Opus 4.8) emit a signed `thinking`
block on assistant turns that also carry parallel `tool_use` blocks. Anthropic
signs that block against the full, original turn content.

When a parallel tool batch is interrupted before every `tool_result` returns,
`_strip_orphaned_tool_blocks` removes the unanswered `tool_use` on replay — which
mutates the turn. The latest-assistant branch of `_manage_thinking_signatures`
then replays the now-stale signed thinking block verbatim, and Anthropic rejects
the request with a non-retryable HTTP 400:

    messages.N.content.M: `thinking` or `redacted_thinking` blocks in the latest
    assistant message cannot be modified. These blocks must remain as they were
    in the original response.

Because the poisoned turn is rebuilt from the persisted store every turn, the
gateway crash-loops with no self-recovery (a soft session reset does not clear
it). The drifting content index in the error is the changing count of stripped
`tool_use` blocks across rebuilds.

Fix: when orphan-stripping removes a `tool_use` from a turn that also holds a
thinking/redacted_thinking block, flag the turn. `_manage_thinking_signatures`
then demotes every thinking block on that latest turn to a plain text block
(preserving the reasoning text) instead of replaying a signature that can no
longer validate. An intact turn is unaffected — its signed thinking is still
replayed verbatim. The internal flag is stripped before the payload is sent.

Adds two regression tests:
- demotion when an orphaned parallel tool_use is stripped
- control: signed thinking preserved verbatim when nothing is stripped