Python: Support message injection for chat flows #6996

Summary

Design and implement Python support for injecting messages into chat flows, similar in spirit to .NET MessageInjectingChatClient but using Python-native composition.

Motivation

Python chat clients do not use the same wrapper pattern as .NET, so message injection should likely be implemented through middleware or another Python-native mechanism.

Scope

• Design the Python message injection surface.
• Prefer middleware/session-aware composition over wrapping chat clients.
• Preserve predictable ordering and replay behavior.
• Add tests and samples/docs as appropriate.

Related

Related to #6995, which tracks informational-only hosted tool calls.