fix: preserve TextMentionTermination.sources across serialization round-trip

[HumphreySun98]

Why are these changes needed?

TextMentionTermination accepts a sources argument that restricts which
message sources are checked for the termination text:

term = TextMentionTermination("stop", sources=["assistant"])

However TextMentionTerminationConfig only stored text, and
_to_config/_from_config only serialized/restored text. So a condition
configured with sources silently loses that restriction after a declarative
dump_component() / load_component() round-trip, and the reloaded condition
then fires on any source (including user or other agents) — a behavior
change on save/load.

The sibling SourceMatchTermination already round-trips its sources field
correctly (and the existing declarative test asserts it), which makes this an
inconsistency rather than an intentional omission.

This PR adds sources to TextMentionTerminationConfig and serializes/restores
it in _to_config/_from_config, with a regression test covering both the
sources-restricted and unrestricted cases.

Related issue number

N/A — found during review. Fixes a serialization round-trip data loss;
SourceMatchTermination is the correct reference behavior.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/ (none required for this change).
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed (ran ruff, mypy, and the relevant pytest locally).

[ErenAta16]

Ran the round-trip test — passes for both the sources-restricted and unrestricted cases. Worth noting this brings TextMentionTermination in line with SourceMatchTermination, which the PR description says already round-trips sources correctly — so this was an inconsistency between two conceptually similar termination conditions rather than a one-off oversight, which matches what I'd expect from how these config classes tend to drift when a field gets added to the runtime class but not mirrored into _to_config/_from_config.