fix(ast): preserve IS NOT NULL and ClickHouse parametric aggregate params in output

[basili4-1982]

Summary

Two separate renderer bugs cause silent data-loss when serializing a parsed
query back to SQL via tree.SQL() / ast.*.SQL():

1. IS NOT NULL renders as IS NULL

The parser stores the operator as "IS NULL" and uses the Not flag to
disambiguate IS NOT NULL (pkg/sql/parser/expressions_operators.go).
BinaryExpression.SQL() (pkg/sql/ast/sql.go) returned the operator string
unchanged and ignored the Not flag in the null branch, inverting the
predicate. A query like WHERE email IS NOT NULL came back as
WHERE email IS NULL.

2. ClickHouse parametric aggregates drop their params group

FunctionCall.SQL() rendered arguments but never emitted Parameters
(field added for ClickHouse fn(params)(args) in #487), turning
quantileTDigest(0.95)(value) into quantileTDigest(value) — the 0.95
parameter was lost on round-trip.

Changes

• pkg/sql/ast/sql.go:
  • BinaryExpression.SQL(): honour Not in the IS NULL branch.
  • FunctionCall.SQL(): render Parameters in their own parenthesis group
    before arguments.
• pkg/sql/ast/is_not_null_test.go: new regression test (exact SQL assert).
• pkg/sql/ast/roundtrip_test.go: added IS NOT NULL roundtrip case.
• pkg/sql/parser/clickhouse_parametric_test.go: new
  TestClickHouseParametricAggregates_Render regression test.

Test plan

• go test -short ./pkg/sql/ast/ ./pkg/sql/parser/ ./pkg/gosqlx/ — all green
• New regression tests fail before the fix, pass after
• Roundtrip (SELECT ... WHERE email IS NOT NULL) idempotent
• ClickHouse quantileTDigest(0.95)(value) round-trips exactly

Note: TestPerformanceRegression fails in this environment against its
baseline (5 cases, SimpleSelect/ComplexQuery/WindowFunction/CTE/INSERT) — all
unrelated to these changes; the degraded queries touch none of the modified
code paths.
Fixes #530 (IS NOT NULL) and #531 (ClickHouse parametric aggregates params).

[vercel]

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[basili4-1982]

Проверено, готово к мержу.

• mergeable: MERGEABLE — без конфликтов с main (main ещё на 31b5b1c).
• Тесты зелёные: go test -short ./pkg/sql/ast/ ./pkg/sql/parser/ ./pkg/gosqlx/ (включая новые TestRenderIsNotNullPreservesNot и TestClickHouseParametricAggregates_Render).
• Единственный красный check — Vercel (Authorization required to deploy): это деплой форка, не связан с кодом; требует авторизации со стороны мейнтейнера. Apply labels и GitGuardian проходят.

Прошу ревью/мерж. Если Vercel блокирует — можно задеплоить из ветки репо или исключить его для PR-форков.