TextMessageTerminationCondition for agentchat

[EItanya]

Why are these changes needed?

Reasons are described in #5732

Related issue number

Closes #5732

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[EItanya]

@ekzhu

[EItanya]

Hey @ekzhu, I found a bit of a problem with this setup. The initial task that a chat begins with is passed through the system as TextMessage with a source set to user and that ends up terminating the chat. I have a local version which includes a config of exclude_sources: list[str] = [] which allows the user to skip certain sources if they want. But it may just be worth always skipping the user source in this case?

Edit:
Just to confirm that once I added this it worked as expected.

[ekzhu]

As describe in the issue #5732 we should have a source parameter for the condition:

TextMessageTermination(source="assistant")

So we can filter by source.

[EItanya]

Ok, pushed the requested impl