Add ChangedEvent signal tests for SettingsVariable

[Copilot]

The ChangedEvent<T> API introduced for changed_signal() and staged_signal() (carrying oldValue_/newValue_) had no test coverage verifying the event payload was correct.

Changes

• ChangedEventSetValue — asserts both changed_signal and staged_signal emit the correct old/new values on a successful SetValue, and that neither fires on a failed (out-of-range) call.
• ChangedEventSetValueToDefault — asserts both signals emit the correct old and new default values after SetValueToDefault.
• ChangedEventStageAndCommit — asserts StageValue fires only staged_signal (not changed_signal), and that Commit fires both with the expected values.

intVariable.changed_signal().connect(
   [&](const ChangedEvent<int64_t>& event) { lastChangedEvent = event; });
intVariable.staged_signal().connect(
   [&](const ChangedEvent<int64_t>& event) { lastStagedEvent = event; });

ASSERT_TRUE(intVariable.SetValue(30));
EXPECT_EQ(lastChangedEvent->oldValue_, 20);
EXPECT_EQ(lastChangedEvent->newValue_, 30);

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