Python: types: ChatMessage.__init__ signature - make contents positional

[eavanvalkenburg]

Consider changing ChatMessage.init signature to make contents a positional argument and potentially add a messages: str parameter.

File: python/packages/core/agent_framework/_types.py
Location: ChatMessage class, init method (Lines ~2424-2430)

Proposed change:

• Make contents a positional argument
• Consider adding messages: str parameter

This would enable simpler ChatMessage construction:
\\python

Before:

user_message = ChatMessage(
role="user",
contents=[TextContent(text="What do you see?"), UriContent(...)],
)

After:

user_message = ChatMessage("user", ["What do you see?", UriContent(...)])
\\

Extracted from #3311
Parent issue: #3575