feat: prevent connection leak during BeginTx context cancellation

[laurentketterle-hub]

Summary

Prevent connection leak to pool when context is cancelled during BeginTx().

Changes

• Added SafeBeginTx() wrapper with proper context handling
• Goroutine-based BeginTx with timeout and cancellation recovery
• Automatic rollback on context cancellation to return connection to pool
• Graceful cleanup when context cancelled with fallback timeout

Tests

• Unit tests for cancelled context
• Unit tests for timeout scenarios
• Integration test for pool connection recovery
• Pool stats verification after cancellation

CI

• Added Go CI workflow with test, vet, and build checks

Closes #2