Add Dialect::supports_cte_materialization and enable it for DuckDB

[adriangb]

WITH c AS [NOT] MATERIALIZED (...) parses only with PostgreSqlDialect, because the parser uses a hard-coded dialect check. DuckDB also accepts this syntax, and custom dialects cannot opt in to it.

let sql = "WITH c AS MATERIALIZED (SELECT 1) SELECT * FROM c";
Parser::parse_sql(&DuckDbDialect {}, sql) // Err: Expected: (, found: MATERIALIZED

This PR replaces the check with a Dialect::supports_cte_materialization method that is off by default. The method returns true for PostgreSQL and DuckDB. GenericDialect still rejects the syntax. Tests check the AST value, the SQL round trip, and the rejection by every dialect that does not opt in.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.25%. Comparing base (c983513) to head (420fab6).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2651   +/-   ##
=======================================
  Coverage   81.24%   81.25%           
=======================================
  Files          42       42           
  Lines       33901    33910    +9     
  Branches    33901    33910    +9     
=======================================
+ Hits        27543    27553   +10     
  Misses       2792     2792           
+ Partials     3566     3565    -1     

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[adriangb]

cc @LucaCappelletti94

[Copilot]

🟢 Approval recommended

The focused change preserves defaults and includes relevant coverage, with no blocking issues identified; tests were not executed.

No open issues 🎉

What changed in this PR

Makes CTE materialization support configurable by dialect, enabling DuckDB while preserving existing defaults.

Changes:

• Adds Dialect::supports_cte_materialization, enabled for PostgreSQL and DuckDB.
• Replaces the parser’s hard-coded PostgreSQL check.
• Adds AST, round-trip, and unsupported-dialect rejection tests.

File	Description
tests/​sqlparser_common.rs	Tests materialization directives and rejection.
src/​parser/​mod.rs	Uses the dialect capability flag.
src/​dialect/​postgresql.rs	Preserves PostgreSQL support.
src/​dialect/​mod.rs	Defines the capability and test-wrapper delegation.
src/​dialect/​duckdb.rs	Enables DuckDB support.

_{Review effort: Balanced}

[LucaCappelletti94]

You should add this DuckDB red test, because nothing fails if DuckDbDialect::supports_cte_materialization returns false. DuckDB then moves into the rejecting set and parse_cte_materialization still passes. With this line, that regression fails with Expected: (, found: MATERIALIZED.

Suggested change

	let query = all_dialects_where(|d| d.supports_cte_materialization()).verified_query(&sql);
	TestedDialects::new(vec![Box::new(DuckDbDialect {})]).verified_query(&sql);
	let query = all_dialects_where(|d| d.supports_cte_materialization()).verified_query(&sql);

[LucaCappelletti94]

LGTM! @adriangb if you have time, please do review some of the other PRs.

cc @alamb