fix(flow): detect tool calls after streamed text

[ruirui6946]

Summary

• scan streamed model output until a tool call or EOF
• apply the same default checker behavior to ReAct and host multi-agent
• update configuration docs and add regression coverage for text-before-tool-call streams

Why

Some models emit text or reasoning chunks before the tool-call chunk. The previous default checker stopped at the first non-empty text chunk, so the agent incorrectly ended instead of executing the tool.

Fixes #892

Validation

• go test ./flow/agent/react ./flow/agent/multiagent/host
• go test ./flow/agent/...

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