fix: web search on DeepSeek and other reasoning modelsfeature/external-providers

A searched turn on a cloud reasoning model ended at the thinking: the
model emitted the tool call, but finish_reason: "tool_calls" landed on
the same SSE line as the include_usage block, so the single-event parser
returned that line as a usage chunk and the loop never saw the tool
finish. The parser now emits every event a line carries, so the search
fires.

Also in this change:
- Replay each round's reasoning_content on the assistant tool-call
  message, which interleaved-thinking models require to keep going.
- Add a per-provider replay_reasoning option (DeepSeek, SiliconFlow
  GLM-4.7+) to carry prior turns' reasoning_content when search is on.
- Add an on-demand diagnostic log gated by $LLAMACHAT_DEBUG_LOG.
- Record provider, usage_json and reported_cost_usd per reply, so a
  searched turn keeps every round's billed usage for external consumers.

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