Add before_acquire passthrough and idle-ping shorthand to ConnectOptions

[tyt2y3]

What

Adds first-class access to SQLx's before_acquire pool hook, plus a shorthand for the common "only ping stale connections" pattern. Previously before_acquire was reachable only through the raw map_sqlx_<db>_pool_opts escape hatch, where you had to hand-write the idle-ping logic and couldn't compose with anything SeaORM set.

API (all additive)

• ConnectOptions::test_before_acquire_if_idle_for(Duration) — DB-agnostic shorthand. Pings a pooled connection before handing it out only once it has been idle for at least the given duration, instead of on every acquire (test_before_acquire). Calling it sets test_before_acquire(false).
• map_sqlx_postgres_before_acquire / map_sqlx_mysql_before_acquire / map_sqlx_sqlite_before_acquire — per-backend passthrough of SQLx's before_acquire callback (per-DB because it operates on the raw &mut DB::Connection).
• get_test_before_acquire / get_test_before_acquire_if_idle_for getters.

How it composes

SQLx's before_acquire slot replaces rather than composes and offers no getter to read it back, so when both the shorthand and a user callback are set they cannot be combined by the caller. A generic ConnectOptions::apply_before_acquire<DB> helper composes them into one closure — idle-ping first, then the user callback — and is called once per driver with that backend's concrete callback. When neither option is set it returns the pool options untouched, so pools built by existing users are byte-for-byte unchanged.

The shorthand alone reduces exactly to:

opts.test_before_acquire(false).before_acquire(|conn, meta| Box::pin(async move {
    if meta.idle_for >= threshold { conn.ping().await?; }
    Ok(true)
}))

Non-breaking

No existing signature, default, or behavior is changed, renamed, or removed. The per-driver insertion is a no-op unless the new options are opted into.

Limitations

Applies only to pools built via Database::connect. The from_sqlx_<db>_pool constructors adopt a pre-built pool and bypass ConnectOptions entirely; configure before_acquire on your own PoolOptions there.

Tests

• Three deterministic construction/composition unit tests in sqlx_common.rs (gated to sqlx-postgres).
• Two doctests on the new methods.
• Verified compiling under sqlx-postgres, sqlx-mysql, sqlx-sqlite, sqlx-all, and --no-default-features; clippy and rustfmt clean on the changed files.