feat: Add FDv2 polling sources to the client

[beekld]

Summary

Adds the FDv2 polling sources -- a one-shot polling initializer and an interval-based polling synchronizer -- over a shared request builder and response handler that feed the response through the protocol handler and the changeset translation.

• The evaluation context travels base64url-encoded in the request path (default) or in a POST body. The FDv1 REPORT option is not carried forward.
• The synchronizer sends the selector's state as the basis query parameter for deltas, with a poll-interval floor to avoid hammering the service.
• The environment id and any FDv1 fallback directive are read from the response headers. A directive carried on a goodbye message takes precedence.

Design note: this client sends no conditional-request validator (If-None-Match/ETag), unlike the Flutter and Android clients, which track a per-context ETag and act on real 304 responses. The spec lists omitting If-None-Match entirely as an accepted way to avoid the cross-context ETag hazard, and since every SDK already sends the basis selector for deltas, dropping the ETag costs little here. Because no If-None-Match is ever sent, a 304 should not occur, so the 304 -> none handling is defensive.

Internal only. The orchestrator that runs these arrives in a later change.

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Note

Overview
Adds FDv2 client polling to the C++ client SDK: shared HTTP request/response plumbing plus two IFDv2* implementations that are not wired into the public client yet.

Shared polling core builds sdk/poll/eval requests from FDv2RequestConfig (context in the path as base64url GET or JSON POST, optional basis and withReasons) and maps responses through FDv2ProtocolHandler into FDv2SourceResult (changesets, goodbye, retries vs terminal errors). It parses X-LD-EnvId and FDv1 fallback headers; body-level goodbye TTL overrides header fallback. 304 is handled as a no-op changeset even though this client deliberately omits If-None-Match.

FDv2PollingInitializer performs a single async poll with cancel-on-Close. FDv2PollingSynchronizer rate-limits repeated polls (minimum interval clamp, respects last poll time) and supports shutdown while waiting or in flight.

Unit tests cover URL construction, response/error/fallback behavior, and synchronizer timing/close semantics. CMakeLists.txt registers the new sources.

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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Invalid URL is treated as retryable

Medium Severity

FDv2PollingSynchronizer never checks whether MakeFDv2PollRequest produced a valid request. An unparseable base_url is sent to the requester, which fails as a transport error and is surfaced as Interrupted. That is a permanent configuration failure, so the orchestrator would keep retrying instead of stopping.

 

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