SQLAlchemy compliance: verify Iceberg UPDATE with target subqueries

[laughingman7743]

Context

Parent issue: #736

The update_where_target_in_subquery requirement is closed.
The compliance suite uses Iceberg tables, but support for an UPDATE target referenced from a subquery can vary with the Athena engine version.

Scope

• Run the relevant SQLAlchemy update tests against Athena engine v3.
• Open update_where_target_in_subquery if Iceberg supports the generated SQL.
• Fix dialect SQL rendering if Athena supports the operation but the emitted statement is invalid.
• Keep the requirement closed when the engine rejects the operation, and document the exact limitation.

Acceptance criteria

• The requirement reflects observed Athena engine v3 behavior.
• Any compiler fix has a focused regression test.
• The targeted Iceberg update test runs with -n 1.
• just lint passes.
• No unrelated DML compliance behavior is changed.