[release/6.0] Microsoft.Data.Sqlite: Avoid re-sending PRAGMA key on pooled connections

[bricelam]

Fixes #28967

Customer Impact

Connection pooling was designed to improve throughput in both high-traffic web scenarios and when using encryption. However, customers reported that when using encryption via SQLCipher, performance had not improved--it was still deriving the encryption key every time a connection was opened. This PR avoids that behavior and ensures the key is only derived once per pooled connection.

Regression?

No.

Risk

Low. This just moves the logic to a place where it will only be executed once per pooled connection instead of every time an ADO.NET connection is opened. The logic will continue to be skipped when not using encryption.

Verification

Manually verified the new perf characteristics. This simple benchmark goes from 5078 ms to 124.

using Dapper;
using Microsoft.Data.Sqlite;

using var connection = new SqliteConnection(
    "Data Source=test.db;Password=Password12!");

connection.Execute(
    "CREATE TABLE IF NOT EXISTS Data(Value)");

for (var i = 0; i < 10; i++)
{
    connection.Execute(
        "INSERT INTO Data VALUES (@Value)",
        new { Value = Random.Shared.Next() });
}