feat: FDv2DataManagerBase for mode switching and data source lifecycle (#1210)

## Summary
- Add `FDv2DataManagerBase` — shared data manager orchestrating FDv2
connection mode switching, state debouncing, and data source lifecycle
- Implements `DataManager` interface with foreground/background mode
support, forced/automatic streaming control, and flush callbacks
- Includes `FlagManager.applyChanges` from #1208 for full/partial/none
flag update semantics

Stacked on #1209 and #1208.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Adds a new shared FDv2 data manager that controls connection-mode
resolution and data source lifecycle, plus updates public `dataSystem`
option shapes; mistakes here could affect SDK initialization,
reconnection behavior, and flag freshness across platforms.
> 
> **Overview**
> Introduces `createFDv2DataManagerBase` (and exported types) as a
shared FDv2 orchestration layer that performs mode
resolution/debouncing, manages `FDv2DataSource` creation/teardown across
identifies and mode changes, tracks selector state, triggers a
background flush callback, and optionally appends a blocked FDv1 polling
fallback synchronizer.
> 
> Updates the public `dataSystem` configuration model by renaming
`initialConnectionMode` to `foregroundConnectionMode` in platform
defaults and by making `automaticModeSwitching` a discriminated union
(`{type:'automatic'}` vs `{type:'manual', initialConnectionMode}`), with
validator support via `validatorOf(..., { is })`; tests are updated and
expanded accordingly, including a large new test suite for
`FDv2DataManagerBase`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0786b64. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
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Author: kinyoklion

packages/shared/sdk-client/__tests__/configuration/Configuration.test.ts

@@ -187,7 +187,7 @@ describe('dataSystem validation', () => {
         getImplementationHooks: () => [],
         credentialType: 'clientSideId',
         dataSystemDefaults: {
-          initialConnectionMode: 'one-shot',
+          foregroundConnectionMode: 'one-shot',
           automaticModeSwitching: false,
         },
       },
@@ -203,61 +203,69 @@ describe('dataSystem validation', () => {
         getImplementationHooks: () => [],
         credentialType: 'clientSideId',
         dataSystemDefaults: {
-          initialConnectionMode: 'one-shot',
+          foregroundConnectionMode: 'one-shot',
           automaticModeSwitching: false,
         },
       },
     );
     expect(config.dataSystem).toBeDefined();
-    expect(config.dataSystem!.initialConnectionMode).toBe('one-shot');
     expect(config.dataSystem!.automaticModeSwitching).toBe(false);
   });
 
   it('validates dataSystem with user overrides applied over platform defaults', () => {
     const config = new ConfigurationImpl(
       // @ts-ignore dataSystem is @internal
-      { dataSystem: { initialConnectionMode: 'polling' } },
+      {
+        dataSystem: {
+          automaticModeSwitching: { type: 'manual', initialConnectionMode: 'polling' },
+        },
+      },
       {
         getImplementationHooks: () => [],
         credentialType: 'mobileKey',
         dataSystemDefaults: {
-          initialConnectionMode: 'streaming',
+          foregroundConnectionMode: 'streaming',
           backgroundConnectionMode: 'background',
           automaticModeSwitching: true,
         },
       },
     );
     expect(config.dataSystem).toBeDefined();
-    expect(config.dataSystem!.initialConnectionMode).toBe('polling');
+    expect(config.dataSystem!.automaticModeSwitching).toEqual({
+      type: 'manual',
+      initialConnectionMode: 'polling',
+    });
     expect(config.dataSystem!.backgroundConnectionMode).toBe('background');
-    expect(config.dataSystem!.automaticModeSwitching).toBe(true);
   });
 
   it('warns and falls back to default for invalid dataSystem sub-fields', () => {
     console.error = jest.fn();
     const config = new ConfigurationImpl(
       // @ts-ignore dataSystem is @internal
-      { dataSystem: { initialConnectionMode: 'turbo' } },
+      { dataSystem: { backgroundConnectionMode: 'turbo' } },
       {
         getImplementationHooks: () => [],
         credentialType: 'clientSideId',
         dataSystemDefaults: {
-          initialConnectionMode: 'one-shot',
+          foregroundConnectionMode: 'one-shot',
           automaticModeSwitching: false,
         },
       },
     );
     expect(config.dataSystem).toBeDefined();
-    expect(config.dataSystem!.initialConnectionMode).toBe('one-shot');
     expect(console.error).toHaveBeenCalledWith(
-      expect.stringContaining('dataSystem.initialConnectionMode'),
+      expect.stringContaining('dataSystem.backgroundConnectionMode'),
     );
   });
 
   it('does not deep-validate dataSystem when dataSystemDefaults is not provided', () => {
     const config = new ConfigurationImpl(
       // @ts-ignore dataSystem is @internal
-      { dataSystem: { initialConnectionMode: 'polling' } },
+      {
+        dataSystem: {
+          automaticModeSwitching: { type: 'manual', initialConnectionMode: 'polling' },
+        },
+      },
       {
         getImplementationHooks: () => [],
         credentialType: 'clientSideId',
@@ -276,7 +284,7 @@ describe('dataSystem validation', () => {
         getImplementationHooks: () => [],
         credentialType: 'clientSideId',
         dataSystemDefaults: {
-          initialConnectionMode: 'one-shot',
+          foregroundConnectionMode: 'one-shot',
           automaticModeSwitching: false,
         },
       },
@@ -288,18 +296,23 @@ describe('dataSystem validation', () => {
   it('validates automaticModeSwitching as a granular config object', () => {
     const config = new ConfigurationImpl(
       // @ts-ignore dataSystem is @internal
-      { dataSystem: { automaticModeSwitching: { lifecycle: true, network: false } } },
+      {
+        dataSystem: {
+          automaticModeSwitching: { type: 'automatic', lifecycle: true, network: false },
+        },
+      },
       {
         getImplementationHooks: () => [],
         credentialType: 'mobileKey',
         dataSystemDefaults: {
-          initialConnectionMode: 'streaming',
+          foregroundConnectionMode: 'streaming',
           automaticModeSwitching: true,
         },
       },
     );
     expect(config.dataSystem).toBeDefined();
     expect(config.dataSystem!.automaticModeSwitching).toEqual({
+      type: 'automatic',
       lifecycle: true,
       network: false,
     });
@@ -312,7 +325,7 @@ describe('dataSystem validation', () => {
         getImplementationHooks: () => [],
         credentialType: 'clientSideId',
         dataSystemDefaults: {
-          initialConnectionMode: 'one-shot',
+          foregroundConnectionMode: 'one-shot',
           automaticModeSwitching: false,
         },
       },