Sync `Environment` via `IScopeObserver` in Hybrid SDK scenarios

[Flash0ver]

Tracking issue for :

• feat: sync environment to the native layers #5365
• fix: fetch default attributes from Scope first #5376

Problem

Per #5376: the SDK has drifted into keeping the Scope largely unpopulated and relying on the Enricher to lazily backfill values like Environment/Release, plus a second pipeline (SetDefaultAttributes) living outside both mechanisms... meaning a user-set Environment on the scope could simply be ignored.

1. At Init: SentryOptions.Environment isn't being pushed into the native SDK, so events originating natively (e.g. a native crash) could carry the wrong/default environment while managed events carried the user's value — inconsistent tagging on the same app.
2. After Init: The scope lets a user overwrite Environment at runtime, but that change isn't propagated.

Note

This issue relates to sentry-native and sentry-cocoa only. sentry-java is addressed separately via:

• Sync Environment via IScopeObserver in Hybrid SDK scenarios (sentry-java) #5387

[linear-code]

NET-739