chore: Minor cleanup

[jsonbailey]

Note

Medium Risk
Touches core data-source synchronization/persistence decisions (selector handling, when to persist, and FDv1 fallback shutdown), which could affect cache/persistent-store behavior if selector semantics are misinterpreted.

Overview
Improves FDv2 polling/streaming behavior around fallback and persistence. PollingDataSourceV2 now only marks data as persist=True when the returned ChangeSet has a defined Selector, and ChangeSetBuilder.no_changes() now returns Selector.no_selector() (instead of None) to make “no selector” explicit; streaming also stops immediately when the server signals FDv1 fallback.

Adds FDv1 polling payload support to the FDv2 polling stack via fdv1_polling_payload_to_changeset, plus associated tests to ensure FDv1 payloads are parsed into a full-transfer changeset but are not persisted. Also includes small cleanups: rename polling endpoint constants for clarity, remove unused RepeatingTask wiring in PollingDataSourceV2, fix a redundant assignment in Store.apply_delta, and correct minor docstring typos.

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