feat: [FD-5562] Adding FDv2 Stream endpoint support in the dev server

[hkotian]

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

Related issues

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Describe the solution you've provided

Piggy backing from the polling fdv2 support, in this PR we are introducing the FDv2 streaming endpoint support.
Like polling endpoints, the initial transfer would either be

1. No-transfer: If there is no change in the payload
2. Or full transfer: Otherwise

An override or sync would also send stream updated in the FDv2 format, as expected.

On a sync the updates will look as follows

event:server-intent
data:{"payloads":[{"id":"fw-test-project","target":15,"intentCode":"xfer-full","reason":"cant-catchup"}]}

event:put-object
data:{"version":15,"kind":"flag","key":"update-flag-4","object":{"key":"update-flag-4","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":107,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"test-flag-22","object":{"key":"test-flag-22","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":6,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"flag-2","object":{"key":"flag-2","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":["B"],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":2,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"flag-A","object":{"key":"flag-A","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":["B"],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":7,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"bool-flag","object":{"key":"bool-flag","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":10,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"pre-req","object":{"key":"pre-req","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":2,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"update-flag-0","object":{"key":"update-flag-0","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":107,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"update-flag-1","object":{"key":"update-flag-1","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":107,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"pre-req-with-string","object":{"key":"pre-req-with-string","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":["second"],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":2,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"update-flag-3","object":{"key":"update-flag-3","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":107,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"update-flag-2","object":{"key":"update-flag-2","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[false],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":107,"deleted":false}}

event:put-object
data:{"version":15,"kind":"flag","key":"flag-3","object":{"key":"flag-3","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":["B"],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":false,"trackEventsFallthrough":false,"debugEventsUntilDate":0,"version":2,"deleted":false}}

event:payload-transferred
data:{"state":"(p:fw-test-project:15)","version":15}

Whereas a single override will emit a single event :

event:server-intent
data:{"payloads":[{"id":"fw-test-project","target":16,"intentCode":"xfer-changes","reason":"update"}]}

event:put-object
data:{"version":16,"kind":"flag","key":"bool-flag","object":{"key":"bool-flag","on":true,"prerequisites":[],"targets":[],"rules":[],"fallthrough":{"variation":0},"offVariation":0,"variations":[true],"clientSideAvailability":{"usingMobileKey":true,"usingEnvironmentId":true},"clientSide":true,"salt":"","trackEvents":true,"trackEventsFallthrough":true,"debugEventsUntilDate":0,"version":11,"deleted":false}}

event:payload-transferred
data:{"state":"(p:fw-test-project:16)","version":16}

Describe alternatives you've considered

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Additional context

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---

Note

Medium Risk
Touches core streaming infrastructure and event payload shapes; mistakes could break SSE clients or cause versioning inconsistencies across polling/streaming updates.

Overview
Adds a new /sdk/stream endpoint that serves Flag Delivery v2 over SSE: it sends an initial FDv2 response based on basis, then pushes incremental flag updates (xfer-changes + put-object + payload-transferred) on overrides and full retransfers on project sync.

To support streaming updates, OverrideEvent and SyncEvent now carry PayloadVersion (propagated from IncrementProjectPayloadVersion), and the generic SSE plumbing is refactored so OpenStream/observers write raw []byte payloads (with Message.ToPayload() used by non-FDv2 streams).

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Related Jira issue: FD-5562: Implement server side streaming endpoint handling in Launchdarkly dev server

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