Add end-to-end tests for ClickHouse core SQL features

Tests for basic ClickHouse functionality: SELECT queries, DML operations
(INSERT/UPDATE/DELETE), DISTINCT, ORDER BY, LIMIT/OFFSET pagination,
NULL handling, and string functions. Each test includes schema,
queries, and generated Go code.

Author: mgilbir

internal/endtoend/testdata/clickhouse_distinct/go/db.go

@@ -0,0 +1,5 @@
+-- name: DistinctDepartments :many
+SELECT DISTINCT department FROM users;
+
+-- name: DistinctMultipleColumns :many
+SELECT DISTINCT department, salary FROM users ORDER BY department;

internal/endtoend/testdata/clickhouse_distinct/go/models.go

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS users
+(
+    id UInt32,
+    name String,
+    department String,
+    salary UInt32
+)
+ENGINE = MergeTree()
+ORDER BY id;

internal/endtoend/testdata/clickhouse_distinct/go/query.sql.go

@@ -0,0 +1,16 @@
+{
+  "version": "2",
+  "sql": [
+    {
+      "engine": "clickhouse",
+      "queries": "query.sql",
+      "schema": "schema.sql",
+      "gen": {
+        "go": {
+          "out": "go",
+          "package": "db"
+        }
+      }
+    }
+  ]
+}