feat: #2228 persist tool origin metadata in run items

[seratch]

This pull request adds ToolOrigin metadata for function-tool-backed run items and preserves it across runtime execution, streaming, and RunState serialization. Resolves #2228

The change introduces a serializable ToolOrigin model in src/agents/tool.py, threads that metadata through tool-call and tool-output item creation in the runtime, and updates RunState schema handling to store and restore the new optional field while keeping older schema versions readable. It also expands exports and MCP/tool helpers where needed so tool-backed items can retain their source information consistently.

Compared with #2242, this version is more durable and less error-prone because it stores only scalar metadata (mcp_server_name, agent_name, agent_tool_name) instead of retaining live MCP server or agent objects. That avoids object pinning and release-time weakref cleanup, makes the metadata safe to round-trip through RunState JSON, and removes identity ambiguity when multiple nested agent tools share the same agent name but have different tool names.

The main behavior change is that tool call/output items can now report whether they came from a plain function tool, MCP-backed function tool, or agent-as-tool path. The serialized state format advances to schema 1.6, but the implementation continues to accept prior released schema versions, so the compatibility risk is limited to consumers that assume the exact emitted snapshot shape rather than using the SDK deserializer.