feat: Add FDv2 data system support to NodeClient (#1775)

SDK-2468

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches core flag data plumbing and connection/offline semantics, but
FDv1 behavior is preserved behind the existing data manager when
`dataSystem` is absent.
> 
> **Overview**
> When **`dataSystem`** is configured, **NodeClient** now wires
**`createFDv2DataManagerBase`** (desktop defaults, transition table,
query params for client-side ID vs mobile) instead of
**`NodeDataManager`**, while leaving the FDv1 path unchanged.
> 
> **Connection mode** is tracked on the client (`_connectionMode`). In
FDv2, **`setConnectionMode`** validates modes, no-ops after close or on
duplicate mode, serializes concurrent transitions (flush before offline,
then event sending + data manager), and **`getConnectionMode` /
`isOffline`** read that state rather than the FDv1 data manager.
> 
> CI runs **FDv2 contract tests** (harness v3) against the same
long-lived test service (`stop_service: 'false'`), with a new
suppressions file and harness updates (**mobile**, **`useMobileKey`**,
extra capabilities). Unit coverage is added in
**`NodeClientFDv2.test.ts`**; **`FDv2ConnectionMode`** is exported from
**`LDCommon`**.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
01ac80d. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: joker23

.github/workflows/node-client.yml

@@ -46,4 +46,13 @@ jobs:
         with:
           test_service_port: 8000
           token: ${{ secrets.GITHUB_TOKEN }}
+          stop_service: 'false'
           extra_params: '--skip-from=${{ github.workspace }}/packages/sdk/node-client/contract-tests/testharness-suppressions.txt'
+      - name: Run contract tests (FDv2)
+        uses: launchdarkly/gh-actions/actions/contract-tests@5adb11fd6953e1bc35d9cf1fc1b4374c464e3a8b # contract-tests-v1.3.0
+        with:
+          test_service_port: 8000
+          token: ${{ secrets.GITHUB_TOKEN }}
+          version: v3
+          branch: v3
+          extra_params: '--skip-from=${{ github.workspace }}/packages/sdk/node-client/contract-tests/testharness-suppressions-fdv2.txt'