fix: send PostgreSQL v3 StartupMessage in proactive IAM refresh probe for MCP

[kgala2]

Summary

Supersedes / extends #1467 to ensure proactive IAM refresh probes actually update the cached IAM access token in Managed Connection Pooling (MCP) PgBouncer for long-lived persistent connections (GoogleCloudPlatform/cloud-sql-proxy#2553).

Why a TLS-only probe is insufficient

When Managed Connection Pooling (PgBouncer) accepts a TLS connection on port 3307:

1. During SBUF_EV_TLS_READY, AutoIAMInit(sbuf) extracts the IAM access token from the ephemeral X.509 certificate extension and stores it in the per-socket client->auto_iam_login_user_credentials->passwd (client->db and client->login_user_credentials are still NULL because the X.509 client certificate does not contain the PostgreSQL database or user).
2. PgBouncer only copies client->auto_iam_login_user_credentials->passwd into the shared pool's client->login_user_credentials->passwd (db->user_tree, which is keyed per (database, username)) inside finish_set_pool(), which is reached only after reading a PostgreSQL v3 StartupMessage (PKT_STARTUP_V3 -> decide_startup_pool -> set_pool(client, dbname, username, ...)).
3. If the probe closes the TLS socket immediately after the TLS handshake without sending a StartupMessage, PgBouncer frees client->auto_iam_login_user_credentials on disconnect without ever updating pool->user_credentials->passwd, causing idle persistent connections to fail after ~1 hour once the initial IAM token expires.

What this PR changes

• Adds @abc.abstractmethod def record_principal(self, user: str, database: str) -> None to ConnectionInfoCache (RefreshAheadCache, LazyRefreshCache, and MonitoredCache).
• Records the (user, db) pair in Connector.connect_async when enable_iam_auth=True.
• Updates _probe_instance_connection (shared by RefreshAheadCache and LazyRefreshCache) to send a PostgreSQL v3 StartupMessage ([int32 length][int32 196608]["user\0<user>\0database\0<database>\0\0"]) for each recorded (user, database) pair over the TLS probe connection, read the initial server authentication response, and send a clean PostgreSQL Terminate packet ('X') before closing.