fix: cast numeric string cache expiry to int for env() compatibility

[derrickobedgiu1]

Related #29

env() always returns strings. When POWER_BI_CACHE_EXPIRY_SECONDS is set in .env, the value arrives as "3600" (string), which the existing is_int() check rejects with an InvalidArgumentException even though the value is valid.

This adds a cast for numeric strings before the integer validation in ConnectorCacheSettings::configureCaching():

if (is_string($expiry) && ctype_digit($expiry)) {
    $expiry = (int) $expiry;
}

Non-numeric strings (e.g. "one-hour") still throw as before. The test suite is updated accordingly: the "throws exception when cache expiry is string" case is replaced with two cases: one confirming "3600" is accepted, one confirming non-numeric strings still throw.