feat: react agent and host multi-agent can be composed directly as AnyGraph

[shentongmartin]

What type of PR is this?

feat

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react agent 和 host multi-agent 支持导出为 AnyGraph，可直接进入编排。

description:

refactor agents to support graph composition

• Allow export graph from react.Agent and host.MultiAgent
• Maintain backward compatibility for direct usage

[github-actions]

⚠️ Breaking API Changes Detected

This pull request contains breaking API changes that may affect compatibility:

uncompatible api changes found: 
github.com/cloudwego/eino/flow/agent/multiagent/host
  Incompatible changes:
  - MultiAgent: old is comparable, new is not
  Compatible changes:
  - (*MultiAgent).ExportGraph: added
  - (*MultiAgent).HostNodeKey: added
  - ConvertCallbackHandlers: added

github.com/cloudwego/eino/flow/agent/react
  Incompatible changes:
  - Agent: old is comparable, new is not
  Compatible changes:
  - (*Agent).ExportGraph: added

Required Actions:

1. Please review these changes carefully
2. Update the user documentation to reflect these API changes
3. Add a note in the changelog about these breaking changes

[github-actions]

📊 Coverage Report:

File coverage threshold (20%) satisfied:	PASS
Package coverage threshold (30%) satisfied:	PASS
Total coverage threshold (83%) satisfied:	PASS
Total test coverage: 83.6% (3269/3911)

[CLAassistant]

All committers have signed the CLA.

[shentongmartin]

⚠️ Breaking API Changes Detected

This pull request contains breaking API changes that may affect compatibility:

uncompatible api changes found: 
github.com/cloudwego/eino/flow/agent/multiagent/host
  Incompatible changes:
  - MultiAgent: old is comparable, new is not
  Compatible changes:
  - (*MultiAgent).ExportGraph: added
  - (*MultiAgent).HostNodeKey: added
  - ConvertCallbackHandlers: added

github.com/cloudwego/eino/flow/agent/react
  Incompatible changes:
  - Agent: old is comparable, new is not
  Compatible changes:
  - (*Agent).ExportGraph: added

Required Actions:

1. Please review these changes carefully
2. Update the user documentation to reflect these API changes
3. Add a note in the changelog about these breaking changes

The breaking API detected is for react.Agent and host.MultiAgent to no longer be comparable. Since no one will actually compare these two structs, this will have no impact on existing users. We will proceed to merge and treat the changes in this PR as compatible.