chore: Create FDv2 polling and fallback data source

[jsonbailey]

Note

Introduces FDv2 polling support and FDv1 fallback, wiring them into the data system and enhancing error/metadata handling.

• Adds PollingDataSource with fetch/sync, plus HTTP requesters HTTPPollingRequester (FDv2 /sdk/poll) and HTTPFDv1PollingRequester (FDv1 /sdk/latest-all) with ETag/304 handling and header extraction (x-launchdarkly-env-id, x-launchdarkly-fd-fallback).
• Provides builders PollingDataSourceBuilder and FDv1PollingDataSourceBuilder and exposes them via DataSystem.polling_ds_builder and fdv1_fallback_ds_builder.
• Implements payload parsing helpers: polling_payload_to_changeset (FDv2 events) and fdv1_polling_payload_to_changeset (flags/segments) returning ChangeSets.
• Extends Result to include optional headers; fixes ChangeSetBuilder.no_changes to use Selector.no_selector.
• Adds extensive specs for initializer, synchronizer, and payload parsing, covering success paths, recoverable/unrecoverable errors, fallback signaling, and env-id propagation.

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[keelerm84]

Changes seem okay to me, minus the bot feedback.

[cursor]

HTTP client connections never closed, causing resource leak

Medium Severity

HTTPPollingRequester and HTTPFDv1PollingRequester create persistent HTTP connections via new_http_client (which calls .persistent()) but neither class provides a stop or close method to release these connections. The existing Requestor class follows the pattern of having a stop method that calls @http_client.close. Without cleanup methods, persistent connections holding file descriptors and sockets are never released when the data source is stopped, leading to resource leaks in long-running applications.

Additional Locations (1)

• lib/ldclient-rb/impl/data_system/polling.rb#L294-L310