Snowflake: Add support for ->> (pipe) operator for chaining SQL stmts

[BenSatori]

The ->> (flow/pipe) operator chains multiple SQL statements in Snowflake.

Before: Parser rejected pipe chains like SELECT * FROM t1 ->> SELECT * FROM $1

After: Parser correctly handles:

• Chained statements with ->> operator
• Pipe result references ($1, $2, $3, etc.)
• Round-trip SQL serialization

Example:

SELECT * FROM users WHERE active = true ->> SELECT COUNT(*) FROM $1

Changes:

• Added Statement::Pipe AST variant for chained statements
• Added TableFactor::PipeResultScan for $n result references
• Extended Snowflake dialect with pipe operator support
• Comprehensive test coverage (2 test functions, multiple edge cases)

Snowflake docs:
https://docs.snowflake.com/en/sql-reference/operators-flow#pipe

[LucaCappelletti94]

This looks like a typo

[LucaCappelletti94]

Several cases that I am not sure are currently corrected supported, such as the following ones, should be added to the tests. These are ones from the Snowflake docs:

// Exact documented SHOW shape.
verified_stmt(
    r#"SHOW WAREHOUSES
       ->> SELECT "name", "state", "type", "size" FROM $1"#,
);

// A Snowflake dialect override that currently scans until EOF/semicolon.
verified_stmt("CREATE DATABASE d ->> SELECT 1");

// Existing happy path.
verified_stmt(
    "CREATE TABLE t (id INT)
     ->> INSERT INTO t VALUES (1)
     ->> SELECT * FROM $1"
);

// Error paths.
assert_parse_error("SELECT 1 ->>");
assert_parse_error("SELECT * FROM $0");

// GenericDialect ambiguity.
verified_generic_expr("SELECT payload ->> 'name'");
verified_generic_pipe("SELECT 1 ->> SELECT 2");

[LucaCappelletti94]

Here you are currently accepting $0, maybe replace with:

if let Ok(index @ 1..) = index_str.parse::<u64>() {

[LucaCappelletti94]

What is this even meant to test? Also, GenericDialect would generally be a dialect that supports all syntaxes, and should therefore likely also support this one.