Internal refactor adds sendBatch seam for partitioned queues

1.40.2

Patch Changes

• #4105 203284a Thanks @eli-r-ph! - Extract the /batch/ submission out of _flush() and sendImmediate() into a single overridable protected sendBatch() seam on PostHogCoreStateless, and widen requestTimeout/historicalMigration to protected. Add an overridable queue-route seam (getQueueRouteKey, persistedQueueKeyForRoute, getActiveQueueRoutes) so a subclass can partition events across independent queues that batch, flush, retry, and persist separately, plus an AiQueue persisted-property key and a route argument on sendBatch. This is an internal, behavior-preserving refactor — with the default single route the enqueue/flush/shutdown/reset paths are byte-identical (v0 request shape, retry, 413 handling, and error surfacing unchanged), so browser and React Native are unaffected. Groundwork for opt-in Capture V1 support in posthog-node, where $ai_* events stay on the legacy transport isolated from the V1 route. (2026-07-11)