Fix serialization unit tests to avoid ORM side effects in --skip-db-tests mode

[holmuk]

In airflow-core/tests/unit/serialization/test_serialized_objects.py, some ORM models (TaskInstance, DagRun, Connection) were instantiated at module import time. In --skip-db-tests mode, Airflow intentionally disables DB setup, so these import-time ORM side effects triggered SQLAlchemy mapper initialization and caused collection-time failures.

This PR resolves this issue:

• Removed unused import-time ORM objects from test_serialized_objects.py to make --skip-db-tests deterministic
• Replaced the global TaskInstance test payload in test_serialize_deserialize parameters with global TaskInstance datamodel
• Moved Connection serialization coverage into dedicated @pytest.mark.db_test tests (test_serialize_deserialize_connection)
• Marked test_backcompat_deserialize_connection as db_test because it creates ORM Connection class

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[Dev-iL]

To be safe, I added a tag to run more tests to make sure nothing unrelated broke as a result of these changes. These will run on the next rebase or after close/reopen.