fix: observability instance already exists

[mario-launchdarkly]

Note

Prevents multiple runtime initializations of Observability and enables identity checks for safe setup.

• Make Observe class-bound by adding AnyObject to allow === identity checks
• Convert ObservabilityClientFactory.noOp to a static singleton and use it as the default in LDObserve
• Add guard in Observability.register to compare LDObserve.shared.client === ObservabilityClientFactory.noOp and throw PluginError.observabilityInstanceAlreadyExist if already initialized
• Introduce PluginError with user-facing error description

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