ci: limit test threads to prevent pool exhaustion (#7852)

* ci: limit test threads to 16 to prevent postgres pool exhaustion

Running all tests with unlimited parallelism exhausts postgres
max_connections (default 100), causing sqlx::test databases to fail
setup and producing spurious RowNotFound errors.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: CI ruby env vars and mysql test binary coercion

- Ruby executor reads RUBY_BUNDLE_PATH and RUBY_GEM_PATH but CI was
  setting BUNDLE_PATH and GEM_PATH, causing "Executable bundle not
  found on worker" errors.
- MySQL test CAST(CONCAT(...) AS CHAR) returns binary type when param
  is bound as bytes. Use CONVERT(? USING utf8mb4) to ensure character
  result type.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/backend-test.yml

@@ -221,4 +221,4 @@ jobs:
         run: |
           deno --version && bun -v && node --version && go version && python3 --version && php --version && ruby --version && pwsh --version && dotnet --version
           cd windmill-duckdb-ffi-internal && ./build_dev.sh && cd ..
-          DENO_PATH=$(which deno) BUN_PATH=$(which bun) NODE_BIN_PATH=$(which node) GO_PATH=$(which go) UV_PATH=$(which uv) PHP_PATH=$(which php) COMPOSER_PATH=$(which composer) RUBY_PATH=$(which ruby) BUNDLE_PATH=$(which bundle) GEM_PATH=$(which gem) POWERSHELL_PATH=$(which pwsh) DOTNET_PATH=$(which dotnet) cargo test --features enterprise,deno_core,duckdb,license,python,rust,scoped_cache,parquet,private,private_registry_test,csharp,php,ruby,mysql,quickjs --all -- --nocapture
+          DENO_PATH=$(which deno) BUN_PATH=$(which bun) NODE_BIN_PATH=$(which node) GO_PATH=$(which go) UV_PATH=$(which uv) PHP_PATH=$(which php) COMPOSER_PATH=$(which composer) RUBY_PATH=$(which ruby) RUBY_BUNDLE_PATH=$(which bundle) RUBY_GEM_PATH=$(which gem) POWERSHELL_PATH=$(which pwsh) DOTNET_PATH=$(which dotnet) cargo test --features enterprise,deno_core,duckdb,license,python,rust,scoped_cache,parquet,private,private_registry_test,csharp,php,ruby,mysql,quickjs --all -- --nocapture --test-threads=16

backend/tests/worker.rs

@@ -1346,7 +1346,7 @@ async fn test_mysql_job(db: Pool<Postgres>) -> anyhow::Result<()> {
 
     let content = r#"
 -- ? name (varchar)
-SELECT CAST(CONCAT('hello ', ?) AS CHAR) AS result;
+SELECT CONCAT('hello ', CONVERT(? USING utf8mb4)) AS result;
 "#
     .to_owned();