feat: add multiagent hooks, add serialize & deserialize function to multiagent base & agent result

[JackYPCOnline]

Description

This RP is a split fom Multiagent session persistent for reviewer friendly.
This RP added multiagent hooks, serialize & deserialize function to multiagent base & agent result
For more context, see origin PR:#900

Related Issues

Documentation PR

Type of Change

Bug fix
New feature
Breaking change
Documentation update
Other (please describe):

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• [x ] I ran hatch run prepare

Checklist

• [x ] I have read the CONTRIBUTING document
• [x ] I have added any necessary tests that prove my fix is effective or my feature works
• [x ] I have updated the documentation accordingly
• [x ] I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• [x ] My changes generate no new warnings
• [x ] Any dependent changes have been merged and published

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[codecov]

Codecov Report

❌ Patch coverage is 63.73626% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/multiagent/base.py	35.41%	30 Missing and 1 partial ⚠️
src/strands/agent/agent_result.py	80.00%	1 Missing and 1 partial ⚠️

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