fix: Send the FDv2 polling selector as the "basis" query parameter

[jsonbailey]

What

The FDv2 (data system v2) polling requester (HTTPPollingRequester) was attaching the current payload selector's state to the poll request under the query-parameter name selector. The protocol requires this parameter to be named basis. Because the server silently ignores unknown query parameters, it never received the SDK's current basis, so every poll returned a full payload and delta polling never engaged.

Fix

Rename the polling query parameter from selector to basis (the value — the selector's state — is unchanged, and it is still only sent when a selector is present). This matches:

• The CSFDV2 spec, which requires the basis query parameter carrying the state from the most recent payload-transferred event.
• The Go server SDK (params["basis"] = selector.State()) and the recently fixed Python SDK.
• Ruby's own FDv2 streaming requester, which already sends basis.

FDv1 polling (HTTPFDv1PollingRequester) does not send a selector/basis and is unaffected.

Testing

This was surfaced by a new sdk-test-harness FDv2 polling basis contract test. The existing polling unit specs (polling_synchronizer_spec.rb, polling_initializer_spec.rb, 39 examples) use mock requesters and continue to pass; none asserted the query-string parameter name, so no unit test change was needed.

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Note

Overview
FDv2 polling now sends the current selector state on poll requests using the basis query parameter instead of selector, matching the CSFDV2 protocol and aligning with FDv2 streaming and other SDKs.

The value (selector.state) and the condition (only when a defined selector exists) are unchanged. FDv1 polling does not send this parameter and is unaffected.

Without this fix, the server ignored the unknown selector parameter, so polls always returned full payloads and delta polling never engaged.

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