feat(ast,parser): round-trip ClickHouse SAMPLE and ORDER BY WITH FILL

[basili4-1982]

Summary

ClickHouse SAMPLE and ORDER BY … WITH FILL are parsed but dropped on render
(they do not round-trip), silently losing query semantics. This PR models both and
makes them round-trip.

Root cause

• SampleClause existed on SelectStatement but had no SQL() render → SAMPLE lost.
• ORDER BY … WITH FILL FROM x TO y STEP z was skipped by the parser
  (skipClickHouseWithFillTail) and not stored.

Changes

• pkg/sql/ast/ast_clauses.go: new WithFillClause (From/To/Step); added
  OrderByExpression.WithFill *WithFillClause.
• pkg/sql/parser/select_clauses.go: ClickHouse WITH FILL tail is now parsed into
  WithFillClause instead of being skipped.
• pkg/sql/ast/sql.go:
  • SampleClause.SQL() renders SAMPLE N[/D] [OFFSET M[/K]];
  • SelectStatement.SQL() emits SAMPLE after ARRAY JOIN/before PREWHERE;
  • orderBySQL() / withFillSQL() render WITH FILL [FROM x] [TO y] [STEP z].
• pkg/sql/parser/clickhouse_sample_withfill_test.go: regression round-trip tests.

Test plan

• New TestClickHouseSampleAndWithFillRoundtrip (sample ratio/fraction/offset,
  with-fill step/full/desc) — all green
• go test -short ./pkg/sql/ast/ ./pkg/sql/parser/ ./pkg/gosqlx/ ./pkg/formatter/ green

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