test: add MySQL backend integration tests

- test_mysql_backend_live.py: live integration tests for MySQL connections
- test_mysql_histogram_sql.py: dialect-specific histogram query tests
- test_mysql_url_builder.py: MySQL connection string construction
- mysql_spinup.sh: Docker spinup script for local MySQL testing

tests/mysql_spinup.sh

@@ -0,0 +1,20 @@
+# start the instance
+docker run -d --rm --name decnet-mysql \
+    -e MYSQL_ROOT_PASSWORD=root \
+    -e MYSQL_DATABASE=decnet \
+    -e MYSQL_USER=decnet \
+    -e MYSQL_PASSWORD=decnet \
+    -p 3307:3306 mysql:8
+
+until docker exec decnet-mysql mysqladmin ping -h127.0.0.1 -uroot -proot --silent; do
+    sleep 1
+done
+
+echo "MySQL up."
+
+export DECNET_DB_TYPE=mysql
+export DECNET_DB_URL='mysql+aiomysql://decnet:decnet@127.0.0.1:3307/decnet'
+
+source ../.venv/bin/activate
+
+sudo ../.venv/bin/decnet api