ref: Improve initialization flow

[limbonaut]

This PR cleans up and restructures initialization flow in preparation for lifecycle methods, extracted from #321.

• Clean up module initialization and class registration.
• Add SentrySDK create_singleton()/destroy_singleton() functions for cleaner code.
• Separate SentrySDK construction from SDK auto initialization.
  • Ensure InternalSDK interface is created early (in constructor).
  • Move startup and initialization logic from constructor into prepare_and_auto_initialize() method.
• Rename InternalSDK initialize() => init() to better align with Sentry terminology.
• Better function names.

These changes make it easier to follow the initialization flow and reduce the risk of errors.

[cursor]

Bug: SDK Initialization Fails on Unsupported Platforms

The _auto_initialize method doesn't correctly update the enabled flag when the SDK is effectively disabled. This occurs on unsupported platforms or when the Android plugin is missing. Although the constructor correctly sets internal_sdk to DisabledSDK in these cases, is_enabled() returns an incorrect true, leading to an inconsistent SDK state.