reconnect() holds the pool slot while sleeping between attempts

`Session::reconnect()` sleeps `retry_interval` between passes and walks all endpoints while still holding its pool slot. With defaults (3 endpoints, 3 attempts, 1s interval) the worst case is ~92s, during which concurrent `acquire()` callers get "pool exhausted" — an error that points away from the real cause.

Fix: when the session is handed out from a pool, skip the between-attempt sleeps (the slot is still held, but the hold time drops to the connect+handshake attempts), or shorten them. Standalone sessions keep the current C#-style pacing.

首先阅读 Session::reconnect，以及将会话交给 acquire 调用方的 pool 路径，重点关注如何识别 pooled 会话和 standalone 会话。验证 pooled reconnect 尝试不会保留尝试之间的 sleep，而 standalone 会话仍保持当前的节奏，并确认并发进行 pool acquisition 时不再因延迟而失败。

由索引模型根据 Issue 内容生成。