fix: Honor urlBuilder on React Native EventSource reconnect

[kinyoklion]

Summary

The shared FDv2 streaming source (StreamingFDv2Base) passes a urlBuilder callback to requests.createEventSource so that every (re)connection recomputes the stream URL and replays the current basis selector. The browser SDK's DefaultBrowserEventSource honors this.

The React Native vendored EventSource (fromExternal/react-native-sse) did not:

• it captured the URL once in its constructor and reused it on every reconnect, and
• PlatformRequests.createEventSource never forwarded urlBuilder.

As a result, after an FDv2 stream reconnect the RN SDK sent an empty basis instead of the previously-known selector. This caused the streaming/fdv2/reconnection state management/{saves,replaces,updates} previously known state contract tests to fail on React Native (the browser SDK passes them).

This change threads urlBuilder through EventSourceOptions -> PlatformRequests.createEventSource -> the RN EventSource, which now recomputes _url from urlBuilder on each _open() -- mirroring the browser implementation.

Verified against sdk-test-harness v3.1.0-alpha.6 on an Android emulator: the three "previously known state" reconnection tests now pass. Added a unit test covering URL recomputation on reconnect.

---

Note

Low Risk
Targeted streaming reconnect behavior with browser parity; covered by a new unit test and existing contract scenarios.

Overview
React Native FDv2 streaming reconnects now recompute the SSE URL on every open, matching the browser SDK.

The vendored RN EventSource adds an optional urlBuilder on EventSourceOptions and invokes it in _open() before each initial and retry connection so dynamic query state (e.g. FDv2 basis) is replayed instead of the constructor URL being reused forever. PlatformRequests.createEventSource forwards eventSourceInitDict.urlBuilder into that implementation.

A unit test asserts urlBuilder runs on first connect and that a forced reconnect uses the updated URL (e.g. ?basis=initial).

^{Reviewed by Cursor Bugbot for commit f0230d4. Bugbot is set up for automated code reviews on this repo. Configure here.}

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