fix: FDv2 fixes surfaced by v3 contract test harness

[beekld]

FDv2 fixes for issues found running the v3 contract test harness against the server SDK.

• Polling URL trailing-slash handling.
• Filter key validation in FDv2 polling and streaming.
• Protocol handler accepts incremental updates after payload-transferred.
• Stream restart on parse failures and protocol errors.
• Unknown SSE event types skipped without parsing.
• Error events preserve protocol state (only accumulated changes cleared).

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Note

Medium Risk
Changes core FDv2 sync behavior (multi-payload streams, error recovery, and reconnect triggers); mistakes could cause missed updates or extra reconnects, but scope is FDv2 data paths with expanded unit tests.

Overview
FDv2 protocol handler now matches streaming incremental semantics: after payload-transferred it stays active in kPartial instead of resetting, so later put/delete + payload-transferred cycles emit partial changesets without a new server-intent. Put/delete are no longer dropped solely because state was inactive; error clears in-flight changes but keeps protocol state so updates can continue. Adds IsKnownEvent for spec-defined ignore behavior.

Polling and streaming build /sdk/poll and /sdk/stream by stripping a trailing-slash empty URL segment (avoids //), and apply the same ValidateFilterKey used elsewhere—invalid filters are omitted with an error log and a full-environment request. MakeFDv2PollRequest takes a Logger for that path.

FDv2 streaming skips unknown SSE event types before JSON parsing, and calls async_restart on malformed JSON or non-server HandleEvent protocol errors so the next connection starts clean.

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