fix: preserve TextMentionTermination sources in config

[maxpetrusenkoagent]

Summary

• Preserve TextMentionTermination sources when dumping/loading component configs.
• Keep old configs compatible by making sources optional and omitted when unset.
• Add a regression test covering source-filtered termination after component round-trip.

Why

Issue #7487 asks for a mission-keeper style goal-integrity node. A practical building block already exists: a read-only integrity agent can emit a drift marker, and TextMentionTermination can restrict that marker to the mission keeper source.

Before this fix, saving/loading the termination condition dropped sources, so any participant could trigger the marker after config round-trip.

Addresses #7487.

Verification

• RED: .venv-agentchat/bin/python -m pytest packages/autogen-agentchat/tests/test_termination_condition.py::test_text_mention_termination_serializes_sources -q failed because config.config.get("sources") was None.
• GREEN: .venv-agentchat/bin/python -m pytest packages/autogen-agentchat/tests/test_termination_condition.py::test_text_mention_termination_serializes_sources -q passed.
• .venv-agentchat/bin/python -m pytest packages/autogen-agentchat/tests/test_termination_condition.py -q passed, 14 tests.
• .venv-agentchat/bin/python -m ruff check packages/autogen-agentchat/src/autogen_agentchat/conditions/_terminations.py packages/autogen-agentchat/tests/test_termination_condition.py passed.
• .venv-agentchat/bin/python -m ruff format --check packages/autogen-agentchat/src/autogen_agentchat/conditions/_terminations.py packages/autogen-agentchat/tests/test_termination_condition.py passed.
• .venv-agentchat/bin/pyright --pythonpath .venv-agentchat/bin/python packages/autogen-agentchat/src/autogen_agentchat/conditions/_terminations.py packages/autogen-agentchat/tests/test_termination_condition.py passed.
• .venv-agentchat/bin/python -m build --wheel packages/autogen-agentchat passed.
• Hermes autoreview of exact diff: CLEAN.

Note: local disk was too constrained for a full workspace uv run sync; verification used a minimal package-scoped venv with autogen-core and autogen-agentchat installed editable.

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