Task LAV-2439: validate SAMPLE / TABLESAMPLE arguments (#2871)

Enforce Snowflake's documented value space for every SAMPLE / TABLESAMPLE
argument position, producing the verbatim Snowflake errors:

- Range/type checks in the sample rewriter: probability 0..=100 and fixed
  size 0..=1,000,000 (001817), seed 0..=2^31-1 (001817) and integer-only
  (001007). Fractional fixed sizes truncate toward zero so the generated
  LIMIT is integral.
- A new pre-pipeline `sample_arg_check` gate emits Snowflake's positioned
  001003 syntax error for a leading `-`, a PERCENT unit, and the
  unsupported BUCKET form (the vendored parser over-accepts all three),
  which also makes the rewriter's BUCKET bail path unreachable.
- The vendored parser now accepts a sampled VALUES list
  (`FROM VALUES (...) SAMPLE (p)`).

Compat tests in test_table_samples.py cover the full value space with
snapshots recorded against real Snowflake.

Co-authored-by: Wojciech Padło <wojciech.padlo@localstack.cloud>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

src/parser/mod.rs

@@ -18050,6 +18050,11 @@ impl<'a> Parser<'a> {
             // where there are no parentheses around the VALUES clause.
             let values = SetExpr::Values(self.parse_values(false, false)?);
             let alias = self.maybe_parse_table_alias()?;
+            // A `VALUES` list can itself be sampled (`... VALUES (…) SAMPLE (p)`),
+            // with the clause following the alias as on any other factor.
+            let sample = self
+                .maybe_parse_table_sample()?
+                .map(TableSampleKind::AfterTableAlias);
             Ok(TableFactor::Derived {
                 lateral: false,
                 subquery: Box::new(Query {
@@ -18065,7 +18070,7 @@ impl<'a> Parser<'a> {
                     pipe_operators: vec![],
                 }),
                 alias,
-                sample: None,
+                sample,
             })
         } else if dialect_of!(self is BigQueryDialect | PostgreSqlDialect | GenericDialect)
             && self.parse_keyword(Keyword::UNNEST)