fix(nim-allographer/baradb): critical client fixes and bug audit

- Export hidden symbols in baradb_client.nim (socket, nextId, readQueryResponse, toString, toBytes)
- Add missing imports in baradb_exec.nim (asyncnet, enums, baradb_query)
- Rewrite toJson() to use wire protocol columnTypes instead of guessing types from strings
- Fix aggregate functions (count, avg, sum) to handle JInt/JFloat/JString correctly
- Fix paginate/fastPaginate discard issues
- Add placeholder arguments for whereIn/whereNotIn/whereBetween/whereNotBetween
- Add placeholder count validation in formatSql()
- Fix getRowPlain() to avoid IndexDefect on empty results
- Harden transactionStart/transactionEnd against server crashes
- Reset placeHolder in insertSql() to avoid stale values
- Fix test_query.nim to use backtick quoting for BaraDB compatibility

Known server bugs documented in BARADB_CLIENT_BUGS.md

clients/nim-allographer/tests/baradb/test_query.nim

@@ -16,11 +16,11 @@ let rdb = baradb
 suite("Baradb query"):
   test("create table raw"):
     let sql = """
-      CREATE TABLE IF NOT EXISTS "users" (
-        "id" SERIAL PRIMARY KEY,
-        "name" VARCHAR(255),
-        "email" VARCHAR(255),
-        "age" INTEGER
+      CREATE TABLE IF NOT EXISTS `users` (
+        `id` SERIAL PRIMARY KEY,
+        `name` VARCHAR(255),
+        `email` VARCHAR(255),
+        `age` INTEGER
       )
     """
     waitFor rdb.raw(sql).exec()