Python: Bug: Multimodal content is dropped during agent handoff in orchestrator

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### Bug Description
In the current implementation of `clean_conversation_for_handoff`, the framework filters the conversation history by exclusively preserving `text` content types and discarding all other content.

While this effectively removes tool-control messages prior to routing, it inadvertently causes multi-modal content (such as `uri` or `data` attachments representing images, audio, or PDFs) to be silently dropped during agent handoffs. As a result, the receiving agent is provided with the text context but loses access to any associated multi-modal files from previous interactions.

### Impact
This behavior affects multi-modal workflows within `SequentialBuilder` and `HandoffAgentExecutor` pipelines. Specifically, it prevents Vision models from accessing images uploaded in prior steps, as the multi-modal context is stripped out during the handoff process.

### Proposed Solution
To resolve this, I propose updating the strict `text` check to a comprehensive allowlist. This approach will safely preserve necessary semantic content (such as `text`, `data`, `uri`, and `hosted_file`) while continuing to reliably filter out runtime and control payloads (such as `function_call`, `usage`, and `error`).

*I have prepared a Pull Request that addresses this issue, complete with updated test coverage in `test_handoff.py`. Thank you for your time and review!*