feat: Add the FDv2 polling data source

[beekld]

Summary

Adds the first concrete FDv2 sources: a polling initializer and synchronizer, both built on the existing HttpTransport.

A few behaviors worth noting:

• The synchronizer clamps its poll interval to a 30-second minimum.
• Requests carry the standard authentication, application tag, and instance-id headers, and send the current selector as the request basis.
• Recoverable HTTP statuses surface as interruptions and unrecoverable ones as terminal errors.

The source also reports an FDv1 fallback directive when a response signals one, which the orchestrator will act on in later work.

NOTE: This PR adds a direct dependency on percent-encoding, but this is low risk, because we already depend on that crate indirectly through other dependencies.

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Note

Overview
Adds the first concrete FDv2 polling sources: a PollingInitializer and PollingSynchronizer that implement the existing Initializer / Synchronizer traits over HttpTransport.

Poll requests go to GET sdk/poll with shared RequestHeaders (auth, user-agent, instance id, optional tags) and an optional selector encoded as the basis query parameter via new URL helpers (percent-encoding dependency). The synchronizer waits between polls and clamps the interval to a 30s minimum.

Responses are mapped to FDv2SourceEvent: 304 yields a no-op change set; 200 bodies are parsed as FDv2 event envelopes through FDv2ProtocolHandler; recoverable HTTP errors become Interrupted, others TerminalError. FDv1 fallback directives are read from X-LD-FD-Fallback / TTL headers (with body precedence on goodbye), via new logic in source.rs.

Dev-only: tokio test-util for paused-clock interval tests; broad unit and mockito e2e coverage for parsing, status handling, selectors, and polling timing.

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