fix: backfill placeholder reasoning_content for bare tool-call messages

Thinking models like kimi/Moonshot and DeepSeek reject any assistant
message carrying tool_calls without a non-empty reasoning_content. When
cross-turn history recovery misses (proxy restart drops the in-memory
cache, ambiguous call_id, or a turn produced no reasoning upstream), the
converted assistant message has none and the whole request fails with
'reasoning_content is missing in assistant tool call message'.

Add a final-stage backfill that runs after all input items are
processed, so genuine trailing reasoning items still attach first and a
placeholder is only injected when none remains. Mirrors the placeholder
behavior in transform::anthropic_to_openai_with_reasoning_content.