sql: render PostgreSQL array literals as ARRAY[...] in unparser

[xiedeyantu]

Which issue does this PR close?

• Closes #.

Rationale for this change

PostgreSQL array literals should be rendered using ARRAY[...] syntax when unparsing SQL. Without this, roundtripped PostgreSQL SQL can lose the dialect-specific array form and emit a plain bracketed array literal instead.

What changes are included in this PR?

• Added a dialect hook to indicate whether array literals should render as ARRAY[...].
• Enabled that behavior for PostgreSqlDialect.
• Updated the SQL unparser to honor the dialect setting when rendering array expressions.
• Adjusted the PostgreSQL roundtrip test to expect ARRAY[1, 2, 3, 4, 5].

Are these changes tested?

Yes. The PostgreSQL roundtrip test in plan_to_sql.rs covers the array literal case and verifies the unparsed SQL now uses ARRAY[...].

Are there any user-facing changes?

Yes. PostgreSQL SQL generated by DataFusion will now emit array literals in ARRAY[...] form instead of plain bracketed form.

[xiedeyantu]

@metegenez Could you please help me review this PR?

[metegenez]

Sure, tomorrow I'll look at it