Fix SQLite cleanup with cached statements

[mcirsta]

Summary

Fix SQLite connection cleanup when Drogon has cached prepared statements.

Sqlite3Connection previously used sqlite3_close() as the connection deleter. If cached prepared statements were still alive when the connection deleter ran, SQLite returned SQLITE_BUSY and left the connection open. The return value was ignored, and no later close attempt was made after the statements were finalized, causing the SQLite connection and related allocations to leak.

This changes the deleter to sqlite3_close_v2(). It marks the connection for deferred destruction, allowing SQLite to release it automatically after the final cached statement has been finalized.

Regression test

The new test:

1. Initializes Drogon and SQLite without creating a cached statement.
2. Records SQLite's memory usage.
3. Creates another client and executes a parameterized query, which enters Drogon's statement cache.
4. Disconnects the client and verifies that SQLite did not retain additional memory.

The test fails with the previous sqlite3_close() behavior and passes with sqlite3_close_v2().

Validation

• Formatted with clang-format 17.0.6
• Debug build with AddressSanitizer
• Full registered test suite: 143/143 passed
• Regression test passed 20 consecutive runs with LeakSanitizer enabled
• Existing SQLite ORM test groups passed:
  • SQLite3Test: 104 assertions
  • SQLite3TransactionTypeTest: 12 assertions
  • SQLite3TransactionTypeLockingTest: 1 assertion
  • DbApiTest: 4 assertions