SQLAlchemy compliance: complete DML result and rowcount coverage

[laughingman7743]

Context

Parent issue: #736

PR #750 enabled SimpleUpdateDeleteTest and corrected result metadata for Athena DML statements.
RowCountTest remains excluded, and InsertBehaviorTest.test_no_results_for_non_returning_insert remains skipped.

Scope

• Audit RowCountTest against Athena Iceberg DML.
• Re-enable rowcount assertions that match the Athena UpdateCount response.
• Gate only assertions that Athena cannot satisfy.
• Re-enable the non-returning insert compliance test if the result metadata fix from Enable SQLAlchemy update and delete compliance tests #750 is sufficient.

Acceptance criteria

• The supported DML compliance tests are no longer deleted or skipped.
• Unsupported rowcount behavior is represented by the narrowest SQLAlchemy requirement gate.
• Regression tests cover any PyAthena behavior changed by the implementation.
• just lint passes.
• Only the affected tests are run locally with -n 1; the complete SQLAlchemy suite is left to CI.

Investigation may conclude that part of RowCountTest is an Athena limitation.
If so, document the engine behavior and keep only those cases gated.