chore: Create FDv2 polling and fallback data source

[jsonbailey]

Note

Introduces a functional polling-based data acquisition path with FDv1 fallback and robust parsing/error handling.

• New PollingDataSource implementing Initializer/Synchronizer with retry logic, recoverable/unrecoverable error handling, and header-driven environment_id/revert_to_fdv1
• Adds HTTP clients: HTTPPollingRequester (FDv2 /sdk/poll) and HTTPFDv1PollingRequester (FDv1 /sdk/latest-all) with ETag support, selector/filter query params, and JSON parsing
• Parsing utilities: polling_payload_to_changeset (FDv2 events) and fdv1_polling_payload_to_changeset (flags/segments) to produce ChangeSets
• Wires builders in DataSystem: polling_ds_builder, fdv1_fallback_ds_builder; streaming_ds_builder remains unimplemented
• Extends Result to carry optional headers; updates ChangeSetBuilder.no_changes to use Selector.no_selector
• Adds specs for initializer behavior, synchronizer state transitions, header handling, and payload parsing

^{Written by Cursor Bugbot for commit fa0bb87. This will update automatically on new commits. Configure here.}

[keelerm84]

Changes seem okay to me, minus the bot feedback.

[cursor]

JSON parse errors lose response headers with fallback signal

Low Severity

The JSON::ParserError rescue blocks in HTTPPollingRequester and HTTPFDv1PollingRequester don't pass response_headers to Result.fail, even though the variable is in scope (assigned on line 259/336 before parsing occurs). All other error paths in these methods correctly include headers. If the server sends a 2xx response with the x-launchdarkly-fd-fallback header but malformed JSON body, the fallback signal is lost and the synchronizer will keep retrying instead of reverting to FDv1.

Additional Locations (1)

• lib/ldclient-rb/impl/data_system/polling.rb#L362-L364