fix: Fix the calculation of the basis parameter for FDv2 streaming. (Does not affect FDv1). (#1165)

The streaming synchronizer was pre-calculating the URL so it didn't
include the selector from the initial poll.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches FDv2 streaming connection setup by changing how the
EventSource URL is built and encoded; a mistake here could break
streaming connections or produce an incorrectly encoded `basis`
parameter.
> 
> **Overview**
> Fixes FDv2 streaming URL construction so the `basis` query parameter
is computed at `start()` time via an optional `selectorGetter`, ensuring
the stream request can include the selector derived from the initial
poll.
> 
> Adds test coverage for including/excluding `basis` based on
`selectorGetter` presence/return value, and updates streaming test
helpers to pass the new option through to `createStreamingBase`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
063d6bc. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: kinyoklion

packages/shared/sdk-client/__tests__/datasource/fdv2/StreamingFDv2Base.test.ts

@@ -430,6 +430,58 @@ it('produces results in order', async () => {
   base.close();
 });
 
+it('includes basis parameter in stream URI when selectorGetter returns a value', () => {
+  const mockEventSource = createMockEventSource();
+  const mockRequests = createMockRequests(mockEventSource);
+
+  const base = createBase(mockRequests, logger, {
+    streamUriPath: '/sdk/stream/eval/ctx',
+    parameters: [{ key: 'auth', value: 'my-key' }],
+    selectorGetter: () => '(p:test:1)',
+  });
+  base.start();
+
+  const uri = mockRequests.createEventSource.mock.calls[0][0];
+  expect(uri).toContain(`basis=${encodeURIComponent('(p:test:1)')}`);
+  expect(uri).toContain('auth=my-key');
+
+  base.close();
+});
+
+it('does not include basis parameter when selectorGetter returns undefined', () => {
+  const mockEventSource = createMockEventSource();
+  const mockRequests = createMockRequests(mockEventSource);
+
+  const base = createBase(mockRequests, logger, {
+    streamUriPath: '/sdk/stream/eval/ctx',
+    parameters: [{ key: 'auth', value: 'my-key' }],
+    selectorGetter: () => undefined,
+  });
+  base.start();
+
+  const uri = mockRequests.createEventSource.mock.calls[0][0];
+  expect(uri).not.toContain('basis');
+  expect(uri).toContain('auth=my-key');
+
+  base.close();
+});
+
+it('does not include basis parameter when no selectorGetter is provided', () => {
+  const mockEventSource = createMockEventSource();
+  const mockRequests = createMockRequests(mockEventSource);
+
+  const base = createBase(mockRequests, logger, {
+    streamUriPath: '/sdk/stream/eval/ctx',
+    parameters: [{ key: 'auth', value: 'my-key' }],
+  });
+  base.start();
+
+  const uri = mockRequests.createEventSource.mock.calls[0][0];
+  expect(uri).not.toContain('basis');
+
+  base.close();
+});
+
 it('close is idempotent', () => {
   const mockEventSource = createMockEventSource();
   const mockRequests = createMockRequests(mockEventSource);

packages/shared/sdk-client/__tests__/datasource/fdv2/streamingTestHelpers.ts

@@ -114,13 +114,15 @@ export function createBase(
     pingHandler?: PingHandler;
     streamUriPath?: string;
     parameters?: { key: string; value: string }[];
+    selectorGetter?: () => string | undefined;
   } = {},
 ) {
   return createStreamingBase({
     requests: mockRequests as any,
     serviceEndpoints,
     streamUriPath: options.streamUriPath ?? '/sdk/stream/eval/test-context',
     parameters: options.parameters ?? [],
+    selectorGetter: options.selectorGetter,
     headers: baseHeaders,
     initialRetryDelayMillis: 1000,
     logger,

packages/shared/sdk-client/src/datasource/fdv2/StreamingFDv2Base.ts

@@ -82,6 +82,7 @@ export function createStreamingBase(config: {
   serviceEndpoints: ServiceEndpoints;
   streamUriPath: string;
   parameters: { key: string; value: string }[];
+  selectorGetter?: () => string | undefined;
   headers: LDHeaders;
   initialRetryDelayMillis: number;
   logger?: LDLogger;
@@ -94,13 +95,17 @@ export function createStreamingBase(config: {
     config.logger,
   );
 
-  const streamUri = getStreamingUri(
-    config.serviceEndpoints,
-    config.streamUriPath,
-    config.parameters,
-  );
   const headers: { [key: string]: string | string[] } = { ...config.headers };
 
+  function buildStreamUri(): string {
+    const params = [...config.parameters];
+    const basis = config.selectorGetter?.();
+    if (basis) {
+      params.push({ key: 'basis', value: encodeURIComponent(basis) });
+    }
+    return getStreamingUri(config.serviceEndpoints, config.streamUriPath, params);
+  }
+
   let eventSource: EventSource | undefined;
   let connectionAttemptStartTime: number | undefined;
   let fdv1Fallback = false;
@@ -256,7 +261,7 @@ export function createStreamingBase(config: {
 
       logConnectionAttempt();
 
-      const es = config.requests.createEventSource(streamUri, {
+      const es = config.requests.createEventSource(buildStreamUri(), {
         headers,
         errorFilter: (error: HttpErrorResponse) => handleError(error),
         initialRetryDelayMillis: config.initialRetryDelayMillis,