fix: SDKE-627 Return true immediately for isInitialized on iOS

[denischilik]

Background

The isInitialized method on iOS was checking the actual initialization state of the native SDK, which could return false during early app lifecycle. This caused issues when Flutter code expected the SDK to be available immediately.

What Has Changed

• Modified isInitialized to return true immediately on iOS instead of checking MParticle.sharedInstance().initialized
• Ensures consistent behavior across platforms where SDK is always considered initialized

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes https://rokt.atlassian.net/browse/SDKE-627