feat: chatagent callbacks and context clone

[fengju0213]

Related Issue

Closes #

Description

• Introduce AgentCallback / AgentEvent interface with 6 lifecycle events (step_started, step_completed, step_failed, tool_started, tool_completed, tool_failed), fired across all execution paths (non-streaming, streaming sync, streaming async)
• Add execution_context (static dict) and execution_context_provider (dynamic callable) to ChatAgent, merged into every event's metadata
• Preserve contextvars context when sync tools are dispatched to thread pools — covers FunctionTool.async_call, _aexecute_tool, _aexecute_tool_from_stream_data, and step() timeout path
• Enhance clone() to accept CloneContext(session_id, execution_context), preserve full runtime config, deep-copy stateful response_terminators, and forward session_id to toolkit clone_for_new_session()

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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