Add file attachment support in Hosted Agent Playground (parity with Model Playground)

[Aenas11]

Agent Playground currently does not support file attachment, while Model Playground does.
For feature parity and practical testing workflows, Agent Playground should also allow attaching files.

Model playground:

Agent playground:

[microsoft-github-policy-service]

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[MuyangAmigo]

Hi @Aenas11 , thanks for bringing this into our attention, I have logged the feature request in our backlog and will get back to you once we have an update.