Add PHPUnit tests workflow for Turso DB

[JanJakes]

Summary

Adds a CI job that runs the SQLite driver unit tests (packages/mysql-on-sqlite) against Turso DB, a Rust reimplementation of SQLite.

The workflow:

• Clones Turso at its latest release tag.
• Builds the turso_sqlite3 crate as a cdylib (a drop-in for libsqlite3's C API).
• Preloads it via LD_PRELOAD so PHP's pdo_sqlite resolves its sqlite3_* symbols against Turso instead of the system libsqlite3.
• Runs PHPUnit.

The job is informational: Turso is in beta with a partially implemented SQLite C API, so failing tests are expected. The step uses continue-on-error: true so the job still succeeds and the test output is visible; we can track compatibility progress over time.

Refs: #204

Test plan

• CI workflow "PHPUnit Tests (Turso DB)" runs on this PR.
• turso_sqlite3 builds successfully from source.
• The "Verify Turso shared library exposes SQLite3 C API" step prints sqlite3_* symbols.
• The "Report SQLite version via Turso preload" step prints a version string (confirming LD_PRELOAD is wired up correctly).
• PHPUnit runs to completion and prints a pass/fail summary.