Add tests for execute_complete success and error handling in AirbyteTriggerSyncOperator

[SameerMesiah97]

Description

This PR adds unit tests for AirbyteTriggerSyncOperator.execute_complete. The tests cover scenarios where the operator completes successfully on non-error events and where an error event triggers an exception.

Rationale

execute_complete is a key part of the deferrable execution flow but was not previously covered by the test suite. The method contains branching logic based on trigger event status that determines whether execution succeeds or fails.

Notes

• The Airbyte connection has been mocked prematurely to accomodate changes in PR Implement execution_timeout semantics for AirbyteTriggerSyncOperator in deferrable mode #64051. These 2 PRs are forwards-compatible i.e. the changes will work irrespective of the merge order.
• Opportunistically updated exception type from AirflowException to RuntimeError to align with agreed consensus and current implementation, as one of the tests asserts that an error is raised.

[SameerMesiah97]

CI failure is unrelated to my changes. It was present in PR #64084, which only changes the spelling of a 'deferrable' in one of the log messages within the execute method of AirbyteTriggerSyncOperator.