test: expand zstd_compression_level test with MySQL 8.4 C API and timing benchmark

The existing test only verified admin variable behavior (default, range,
independence). It did not prove that ZSTD compression was actually
active on the wire. Two new test variants address this:

MariaDB connector path (mysql-zstd_compression_level-t):
- Tests 1-9: unchanged (variable defaults, range validation, independence)
- Tests 10-11: use mysql CLI via execvp with --compression-algorithms=zstd
  and --zstd-compression-level={3,19}. Each call spawns a new process
  (= new connection). Verifies CONNECTION_ID() retrieval and large
  resultset transfer. Skipped if mysql CLI lacks zstd support.

MySQL 8.4 connector path (mysql-zstd_compression_level_libmysql-t):
- Tests 1-9: same as MariaDB path
- Test 10: establishes a ZSTD connection using mysql_options(
  MYSQL_OPT_COMPRESSION_ALGORITHMS, "zstd") and prints mysql_thread_id()
  without running any query.
- Test 11: timing benchmark proving compression is active. Creates a
  query rule (rule_id=1, cache_ttl=60000) to cache all SELECTs, then
  runs a 576-row resultset query 1000 times on a plain connection and
  1000 times on a ZSTD level 22 connection. ZSTD level 22 is extremely
  CPU-intensive: observed 40122ms vs 858ms (46.7x ratio). A ratio >= 3x
  proves compression is really running on the client<->ProxySQL link.

Other changes:
- MySQL_Protocol.cpp: simplify redundant clamping to a simple cast
  (the variable is already validated at SET time)
- Makefile: add build rule for _libmysql variant linking against
  MySQL 8.4 connector
- groups.json: register _libmysql variant for mysql84/90/95-g1
- CLAUDE.md: update testing section with CI script usage and DO NOT list

Author: renecannao

CLAUDE.md

@@ -55,22 +55,39 @@ The same codebase produces three product tiers via feature flags:
 
 Tests use TAP (Test Anything Protocol) with Docker-based backend infrastructure.
 
+### Running TAP tests — DO NOT manually set up Docker containers
+
+**ALWAYS use `run-tests-isolated.bash`**. It handles infrastructure setup, ProxySQL start, test execution, and cleanup. Never manually create Docker networks, start containers, or run init scripts — the runner does all of that.
+
 ```bash
-# Build and run all TAP tests
-make build_tap_tests
-cd test/tap && make
+# Set up infrastructure (backends + ProxySQL container)
+WORKSPACE=$(pwd) INFRA_ID=dev-$USER TAP_GROUP=mysql84-g1 test/infra/control/ensure-infras.bash
 
-# Run specific test groups
-cd test/tap/tests && make
-cd test/tap/tests_with_deps && make
+# Run all tests for a TAP group
+WORKSPACE=$(pwd) INFRA_ID=dev-$USER TAP_GROUP=mysql84-g1 test/infra/control/run-tests-isolated.bash
 
-# Test infrastructure (Docker environments)
-# Located in test/infra/ with docker-compose configs for:
-# mysql57, mysql84, mariadb10, pgsql16, pgsql17, clickhouse23, etc.
+# Build test binaries first (requires proxysql binary)
+make build_tap_tests          # release
+make build_tap_test_debug     # debug
 ```
 
+Available TAP groups are defined in `test/tap/groups/groups.json`. Group names follow the pattern `<infra>-g<N>` (e.g., `mysql84-g1`, `legacy-g2`, `pgsql16-g1`).
+
+### DO NOT
+
+- **DO NOT** manually create Docker networks (`docker network create`)
+- **DO NOT** manually start containers (`docker start`, `docker run`)
+- **DO NOT** run `docker-compose-init.bash` directly — use `ensure-infras.bash`
+- **DO NOT** symlink build artifacts between worktrees — build in each worktree separately
+- **DO NOT** copy source files between worktrees or repos
+- **DO NOT** run `cd test/tap/tests && make` and expect tests to pass without infrastructure
+
+### Test file conventions
+
 Test files follow the naming pattern `test_*.cpp` or `*-t.cpp` in `test/tap/tests/`.
 
+Test binaries are built via a pattern rule in `test/tap/tests/Makefile`: `make <testname>-t` compiles `<testname>-t.cpp` into `<testname>-t`. No special Makefile target is needed for new tests — just add the `.cpp` file and register it in `groups.json`.
+
 ## Architecture
 
 ### Build Pipeline

lib/MySQL_Protocol.cpp

@@ -2300,7 +2300,7 @@ void MySQL_Protocol::PPHR_SetConnAttrs(MyProt_tmp_auth_vars& vars1, account_deta
 	const uint8_t zstd_compression_level =
 		(vars1.zstd_compression_level > 0 && vars1.zstd_compression_level <= ZSTD_maxCLevel())
 			? vars1.zstd_compression_level
-			: static_cast<uint8_t>(std::min<int>(ZSTD_maxCLevel(), std::max<int>(1, mysql_thread___zstd_compression_level)));
+			: static_cast<uint8_t>(mysql_thread___zstd_compression_level);
 
 	myconn->options.compression_zstd = false;
 	myconn->options.zstd_compression_level = 0;

test/tap/groups/groups.json

@@ -100,6 +100,7 @@
   "mysql-test_ssl_CA-t" : [ "legacy-g1","mysql-auto_increment_delay_multiplex=0-g1","mysql-multiplexing=false-g1","mysql-query_digests=0-g1","mysql-query_digests_keep_comment=1-g1","mysql84-g1","mysql90-g1","mysql95-g1" ],
   "mysql-watchdog_test-t" : [ "legacy-g4","mysql-auto_increment_delay_multiplex=0-g4","mysql-multiplexing=false-g4","mysql-query_digests=0-g4","mysql-query_digests_keep_comment=1-g4","mysql84-g4","mysql90-g4","mysql95-g4" ],
   "mysql-zstd_compression_level-t" : [ "legacy-g1","mysql-auto_increment_delay_multiplex=0-g1","mysql-multiplexing=false-g1","mysql-query_digests=0-g1","mysql-query_digests_keep_comment=1-g1","mysql84-g1","mysql90-g1","mysql95-g1" ],
+  "mysql-zstd_compression_level_libmysql-t" : [ "mysql84-g1","mysql90-g1","mysql95-g1" ],
   "mysql_encode_unit-t" : [ "unit-tests-g1" ],
   "mysql_error_classifier_unit-t" : [ "unit-tests-g1" ],
   "mysql_hostgroup_attributes-servers_defaults-t" : [ "legacy-g1","mysql-auto_increment_delay_multiplex=0-g1","mysql-multiplexing=false-g1","mysql-query_digests=0-g1","mysql-query_digests_keep_comment=1-g1","mysql84-g1","mysql90-g1","mysql95-g1" ],

test/tap/tests/Makefile

@@ -263,6 +263,9 @@ mysql_reconnect_libmariadb-t: mysql_reconnect.cpp $(TAP_LDIR)/libtap.so
 mysql_reconnect_libmysql-t: mysql_reconnect.cpp $(TAP_LDIR)/libtap_mysql8.a
 	$(CXX) -DLIBMYSQL_HELPER8 -DDISABLE_WARNING_COUNT_LOGGING $< -I$(TEST_MYSQL8_IDIR) -I$(TEST_MYSQL8_EDIR) -L$(TEST_MYSQL8_LDIR) -lmysqlclient -ltap_mysql8 -lresolv $(CUSTOMARGS) -o $@
 
+mysql-zstd_compression_level_libmysql-t: mysql-zstd_compression_level-t.cpp $(TAP_LDIR)/libtap_mysql8.a
+	$(CXX) -DLIBMYSQL_HELPER8 -DDISABLE_WARNING_COUNT_LOGGING $< -I$(TEST_MYSQL8_IDIR) -I$(TEST_MYSQL8_EDIR) -L$(TEST_MYSQL8_LDIR) -lmysqlclient -ltap_mysql8 -lresolv $(CUSTOMARGS) -o $@
+
 fast_forward_grace_close_libmysql-t: fast_forward_grace_close.cpp $(TAP_LDIR)/libtap_mysql8.a
 	$(CXX) -DLIBMYSQL_HELPER8 -DDISABLE_WARNING_COUNT_LOGGING $< -I$(TEST_MYSQL8_IDIR) -I$(TEST_MYSQL8_EDIR) -L$(TEST_MYSQL8_LDIR) -lmysqlclient -ltap_mysql8 -lresolv $(CUSTOMARGS) -o $@