A failed product version lookup is cached for the whole connection #112

LibsqlDatabaseMetaData.resolveDatabaseProductVersion catches SQLException and stores "3.0" in cachedDatabaseProductVersion. #94 caches the metadata instance per connection. One network error now pins getDatabaseProductVersion() to "3.0" for the life of the connection. getDatabaseMajorVersion() and getDatabaseMinorVersion() return 3 and 0.

On a closed connection the method also returns "3.0", while getTables and getColumns throw. The cache field is not synchronized.

This is separate from the statement leak in #92.

Expected:

• Do not cache the fallback value. Let the exception propagate, or retry on the next call.
• Throw on a closed connection.
• Add a test with a failing version query followed by a successful one.