Move large Statement arms out of SqlToRel::sql_statement_to_plan_with_context_impl #26301

Is your feature request related to a problem or challenge?

SqlToRel::sql_statement_to_plan_with_context_impl (sql/src/statement.rs#L267-L1649) is a 1,307-line match over ast::Statement, with cognitive complexity 357 and cyclomatic complexity 217. statement.rs has had 39 commits in the last 12 months.

Many arms are already thin dispatchers (explain_to_plan, set_statement_to_plan, describe_table_to_plan, ...), but about a dozen inline large bodies:

• CreateTable (about L312-L652, ~340 lines), mostly destructuring ~70 sqlparser fields followed by a long run of if x { return not_impl_err!(..) }.
• CreateView, Drop, Prepare/Execute, ShowTables/ShowColumns, Insert, StartTransaction (L1268-L1344), CreateFunction (L1372-L1524) and Truncate.

Every sqlparser upgrade that adds a field to CreateTable has to touch this destructuring pattern, which makes the function a recurring merge-conflict and review burden.

Describe the solution you'd like

• Make the match a pure dispatcher where each large arm calls a method on SqlToRel, following the existing describe_table_to_plan pattern: create_table_to_plan, create_view_to_plan, drop_to_plan, prepare_to_plan, execute_to_plan, show_tables_to_plan, start_transaction_to_plan, create_function_to_plan, truncate_to_plan, ...
• For CreateTable, bind the struct by name (Statement::CreateTable(create_table)) and move the unsupported-option checks into a helper such as reject_unsupported_create_table_options(&create_table), so new sqlparser fields don't require editing the match pattern.
• Optionally split statement.rs into submodules by statement family (DDL, DML, utility).

Describe alternatives you've considered

Leave it as is.

Additional context

Line references are pinned to commit 0d79252.