fix(agentchat): persist sources filter in TextMentionTerminationConfig

[xodn348]

Summary

TextMentionTermination has accepted a sources whitelist parameter since its introduction, allowing callers to restrict termination checks to specific agent names. However, TextMentionTerminationConfig — the Pydantic model used for component serialization — only stored the text field. Every serialize → deserialize round-trip (e.g. loading a team from YAML or JSON) silently dropped the sources filter, restoring the "check all messages" behaviour. This meant a component configuration that worked correctly in memory would produce different, broken behaviour after persistence.

One observable consequence (reported in #6826) is that the termination phrase placed in the initial user task prompt triggers immediate termination: the user passes sources=["assistant"] to avoid checking their own prompt, but after loading from config sources is gone and the user message fires the condition instead.

The fix adds sources: List[str] | None = None to TextMentionTerminationConfig, updates _to_config to serialize the value, updates _from_config to pass it back to __init__, and improves the docstring with a concrete guidance note. A round-trip assertion is added to test_declarative_components.py.

Issue

Refs #6826

Local verification

$ cd python/packages/autogen-agentchat
$ python -m pytest tests/test_termination_condition.py tests/test_declarative_components.py::test_termination_declarative -v

============================= test session starts ==============================
platform linux -- Python 3.11.15, pytest-9.0.3, pluggy-1.6.0
asyncio: mode=Mode.STRICT
collected 14 items

tests/test_termination_condition.py::test_handoff_termination PASSED
tests/test_termination_condition.py::test_stop_message_termination PASSED
tests/test_termination_condition.py::test_text_message_termination PASSED
tests/test_termination_condition.py::test_max_message_termination PASSED
tests/test_termination_condition.py::test_mention_termination PASSED
tests/test_termination_condition.py::test_token_usage_termination PASSED
tests/test_termination_condition.py::test_and_termination PASSED
tests/test_termination_condition.py::test_or_termination PASSED
tests/test_termination_condition.py::test_timeout_termination PASSED
tests/test_termination_condition.py::test_external_termination PASSED
tests/test_termination_condition.py::test_source_match_termination PASSED
tests/test_termination_condition.py::test_function_call_termination PASSED
tests/test_termination_condition.py::test_functional_termination PASSED
tests/test_declarative_components.py::test_termination_declarative PASSED

14 passed in 1.17s
=== LOCAL_TEST_PASSED ===

Also ran ruff on the changed files — 1 import-ordering fix applied automatically, 0 remaining errors.

Risk

The only change to the runtime __call__ path is in _to_config / _from_config; in-memory behaviour of existing code that never round-trips through the config is unaffected. Callers who rely on the old (broken) serialisation omitting sources would see the field appear in their config JSON with value null, which is schema-compatible. No breaking API change.

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