fix(node-server-sdk): No FDv1 fallback when using custom datasystem

[joker23]

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

SDK-1802

Describe the solution you've provided

• FDv1 fallback will now always be created
• fixed contract tests for node server sdk so it will work with latest in feat/fdv2
• enabled the basic initializer contract test for node server sdk

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Note

Medium Risk
Touches core SDK data source construction and fallback behavior, which can affect flag delivery and initialization paths; changes are localized but could impact runtime update behavior if misconfigured.

Overview
Fixes FDv2 initialization so an FDv1 polling fallback synchronizer (/sdk/latest-all) is always added, including when dataSystem.dataSource is custom.

Updates contract-test wiring to match the current feat/fdv2 harness: CI now checks out the branch directly, the contract-test client config now accepts lists of synchronizers/initializers and builds custom DataSourceOptions accordingly, and one FDv2 suppression is removed to re-enable the basic initializer test.

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