chore: Remove unused Payload.id field from FDv2 protocol handler

[kinyoklion]

SDK-2005

Summary

• Removes the id field from the internal Payload interface and all tempId plumbing in the protocol handler
• The field was populated from PayloadIntent.id but never read by any downstream consumer
• Wire protocol types (PayloadIntent.id, PayloadTransferred.id, ErrorObject.payload_id) are unchanged

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Note

Low Risk
Low risk refactor that removes an unused field and related plumbing; behavioral changes are limited to slightly different validation/log messages around intent/transfer events.

Overview
Removes the internal FDv2 Payload.id field and the protocol handler’s tempId tracking, so emitted payloads are keyed only by version/state/type/updates.

Updates SDK client/server components and tests to stop constructing or asserting on payload.id (including cache initialization, polling/FDv1 fallback payloads, and FDv2 source result helpers), and simplifies protocol-handler warnings/log messages accordingly.

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@launchdarkly/js-client-sdk size report
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[github-actions]

@launchdarkly/js-client-sdk-common size report
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