fix: FDv2 initializer readiness

[joker23]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Describe the solution you've provided

This PR will:

1. Make it so that a client in FDv2 will only report intialized when it gets a basis + valid selector. This will align the behavior better with the FDv2 initializer specs.
2. Updated the docs for the custom datasystem options with an example for clarity

---

Note

FDv2 now calls the initialization callback only when payload.state is non-empty, with tests added, and docs updated with an example for custom data source options.

• Data sources:
  • In createPayloadListenerFDv2, replace basisReceived with initializedCallback and invoke it only when payload.state !== '' during applyChanges.
• Tests:
  • Update createPayloadListenersFDv2.test.ts to use initializedCallback and assert conditional invocation for non-empty vs empty state; keep existing behavior checks for basis/updates.
• Docs:
  • Expand JSDoc for CustomDataSourceOptions in LDDataSystemOptions.ts with @experimental note and a usage example mirroring default standard options.

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[kinyoklion]

Would the FDv2 fallback also not have a selector? I may just not know how that part works.

[joker23]

From my understanding here, the basis can be consumed without a selector, however that would not be considered a successful initialization from this particular initializer. I think this function might not be named correctly here as from the invocation it is simply a success callback (

js-core/packages/shared/sdk-server/src/LDClientImpl.ts

Line 459 in 8113122

payloadListener = createPayloadListener(dataSourceUpdates, logger, initSuccess);

). @tanderson-ld thoughts?

[tanderson-ld]

I agree there is a bug in the code before this PR as even when payload.basis is false, basisReceived is called.

LDClientImpl is passing in a function called initSuccess. I think at that time I had thought getting a basis (independent of selector) was sufficient to consider it initialized. This would be my thinking at the time since I thought this code path was only hit when basis == true.

Since Casey's requirement is "Basis with a Payload Selector.", should this logic be updated to ensure basis is also true when selector is not null?

The name basisRecieved in basisReceived: VoidFunction = () => {} parameter may need to be adjusted.