fix: Decouple event processor and data source lifecycles

[beekld]

Summary

The SDK's flag data source, event processor, and diagnostic reporter shared a single lifecycle. A terminal HTTP error (a 401) from either the event processor or the flag data source took the whole client offline and overwrote the flag-delivery connection state, regardless of which component actually failed. Terminal non-401 handling was also inconsistent. A streaming 403/404 stopped the stream silently while the client still reported online, and polling retried forever.

Each network component now owns its own error handling. A terminal error scopes to the component that hit it. A data-source error stops only flag delivery, an event-processor error stops only event delivery, and neither affects the other. Event-delivery failures no longer modify ConnectionInformation, and a permanently stopped data source is now observable rather than silently stopping while the client reports online.

This also drops the legacy whole-client shutdown on 401, matching the other LaunchDarkly client SDKs.

Note for reviewers

Previously a terminal data-source error had no consistent observable state. A rejected stream could stop while the client still reported online. Now any terminal data-source error sets the connection mode to .offline with a failure reason. We reuse the existing .offline mode rather than add a dedicated terminal case, because adding a value to the public ConnectionMode enum would be a breaking change for consumers that switch over it exhaustively. The FDv2 spec discourages conflating an auth error with a deliberate offline, but FDv1's ConnectionMode has no error or interrupted value, so .offline is the only signal available.

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Note

Overview
Decouples flag delivery, event reporting, and diagnostics so a terminal HTTP failure in one path no longer forces the whole LDClient offline. The legacy behavior that called internalSetOnline(false) on flag-sync errors (including 401) is removed.

Terminal 4xx handling is centralized via HTTPURLResponse.StatusCodes.isTerminalStatusCode (retries still allowed for 400, 408, 429). SynchronizingError gains httpStatusCode and isTerminal; streaming, polling, diagnostics, and events all use the same rule.

Flag data source: On a terminal sync error, only flagSynchronizer.isOnline is set false, ConnectionInformation records failure and sets currentConnectionMode to .offline, and the client is marked initialized while LDClient.isOnline can stay true. Successful syncs run checkEstablishingStreaming(..., streamingMode:) to move back from .offline to streaming or polling. Event reporter: Non-retriable responses drop events and set EventReporter.isOnline false without touching connection information. Event sync completion no longer routes errors through LDClient.process.

Polling edge cases: While the synchronizer is offline, in-flight poll responses are ignored and the flag etag cache is cleared so the next poll refetches; cached “up to date” shortcuts are skipped when offline.

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