Feat/tool call loop (#6651)

## Why are these changes needed?

This PR addresses critical issues in the AssistantAgent that affect tool
handling:

**Lack of tool call loop functionality**: The agent could not perform
multiple consecutive tool calls in a single turn, limiting its ability
to complete complex multi-step tasks that require chaining tool
operations.

These changes enhance the agent's robustness and capability while
maintaining full backward compatibility through feature flags.

## Related issue number

Closes  #6268

## Checks

- [x] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [x] I've added tests corresponding to the changes introduced in this
PR.
- [x] I've made sure all auto checks have passed.

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Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>