refactor: Update conversation manager interface

[Unshure]

Description

Refactor the ConversationManager so that it can take in an Agent instead of a list of messages. This is to simplify the interface, and better support the new SummarizationConversationManager implementation: #112

Related Issues

#90

Documentation PR

No changes needed

Type of Change

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

Breaking change

Testing

• hatch fmt --linter
• hatch fmt --formatter
• hatch test --all
• Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

Checklist

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

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.