db: cleanup transaction implementation and stabilize connection pool

- Make Transaction fully header-only (remove src/Transaction.cpp)
- Update ConnectionPool implementation for ORM compatibility
- Align CMakeLists with header-only DB primitives

Author: GaspardKirira

CMakeLists.txt

@@ -218,14 +218,12 @@ set(VIX_DB_PUBLIC_HEADERS
 
 set(VIX_DB_SOURCES
   src/pool/ConnectionPool.cpp
-  src/Transaction.cpp
   src/Database.cpp
   src/mig/MigrationsRunner.cpp
   src/mig/FileMigrationsRunner.cpp
   src/Sha256.cpp
 )
 
-
 # MySQL driver sources
 if (VIX_DB_HAS_MYSQL)
   list(APPEND VIX_DB_PUBLIC_HEADERS include/vix/db/drivers/mysql/MySQLDriver.hpp)

src/Transaction.cpp

@@ -75,6 +75,9 @@ namespace vix::db
     for (std::size_t i = 0; i < cfg_.min; ++i)
     {
       auto c = factory_();
+      if (!c || !c->ping())
+        throw DBError("ConnectionPool::warmup: factory returned invalid connection");
+
       idle_.push(std::move(c));
       ++total_;
     }