chore: Provide selector for use as basis on FDv2 data sources

[keelerm84]

Note

Introduces SelectorStore and threads selectors through polling/streaming APIs, FDv2, and tests, including SSE basis query support.

• Core API (datasystem):
  • Add SelectorStore protocol and require selector().
  • Change Initializer.fetch and Synchronizer.sync signatures to accept SelectorStore.
• Polling (datasourcev2/polling.py):
  • Update PollingDataSource.fetch/sync/_poll to use ss.selector() when requesting data.
  • Propagate selector to Requester.fetch; no-change/full/changes handling unchanged.
• Streaming (datasourcev2/streaming.py):
  • Update SseClientBuilder to take (Config, SelectorStore) and build client with dynamic query_params using selector basis.
  • Update StreamingDataSource.sync to pass SelectorStore to client builder.
• FDv2 orchestration:
  • Pass store as SelectorStore to initializers and synchronizers; consume updates unchanged.
• Test data source (TestDataV2 impl):
  • Conform to new interfaces: fetch(ss), sync(ss), stop() (renamed from close).
• Tests/Mocks:
  • Add MockSelectorStore and update all tests to pass a SelectorStore.
  • Adjust streaming builder/test helpers for new signatures and basis query behavior.

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[kinyoklion]

Write lock?

[kinyoklion]

Same question for all the mutations.

[keelerm84]

That had been fixed in a previous PR. Just needed rebasing.

[kinyoklion]

Locking questions inline.