feat!: make Scope.Environment non-nullable

[jamescrosswell]

Summary

Scope.Environment is now declared as string rather than string?.

#5365 made the setter revert null to SentryOptions.Environment, but the property could still return null: a scope whose environment was never set had a null backing field, and SentryOptions.Environment is itself nullable. To make the non-nullable declaration truthful:

• The getter falls back to SettingLocator.GetEnvironment() (options → SENTRY_ENVIRONMENT → production/debug), the same value the Enricher already stamps on events.
• The setter keeps accepting null ([AllowNull], so it still satisfies IEventLike.Environment) and resets to that resolved default.
• Scope.Apply copies only an explicitly set environment between scopes. Otherwise the non-null getter would make ??= never copy onto a cloned scope.

Notes for review

• Breaking change (public API signature), hence targeting version7.
• Side effect: Apply to an event now writes the resolved default environment from the scope rather than leaving it for the Enricher. The value is identical.
• The Populate_RouteData_SetToScope snapshot now includes Environment: production, since serializing a scope reads the resolved value.
• Apply_Environment_Null asserted a target scope's environment stays null, which is no longer possible. It now asserts the options fallback.

Closes #5388

🤖 Generated with Claude Code

[codecov]

Codecov Report

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Additional details and impacted files

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##             version7    #5587   +/-   ##
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