Python: agents: ChatAgent.run messages parameter - allow Contents for multimodal

[eavanvalkenburg]

The ChatAgent.run messages parameter should also allow Contents directly for better multimodal support.

File: python/packages/core/agent_framework/_agents.py
Location: ChatAgent class, run method, messages parameter (Lines ~743-749)

Current: Only allows str | ChatMessage | Sequence[...]
Proposed: Also allow Contents directly

This would enable:
\\python
result = await agent.run([
"What do you see in this image?",
UriContent(uri="https://...", media_type="image/jpeg"),
])
\\

Extracted from #3311
Parent issue: #3575