refactor: centralize test fixtures via ffe-system-test-data submodule

[leoromanovsky]

Motivation

The Node.js SDK currently maintains its own copy of UFC evaluation fixtures in packages/node-server/test/data/. This creates drift when fixtures are updated across SDKs. The ffe-system-test-data repo provides a single source of truth for UFC evaluation test cases shared across all Datadog OpenFeature SDKs.

Changes

• Added ffe-system-test-data as a git submodule at packages/node-server/test/ffe-system-test-data
• Updated flags-v1.spec.ts to read ufc-config.json and evaluation-cases/ from the submodule instead of data/flags-v1.json and data/tests/
• Updated provider.spec.ts to read config from submodule
• Removed the UniversalFlagConfigurationV1Response wrapper — the shared config uses the unwrapped UniversalFlagConfigurationV1 format directly
• Deleted all local fixture copies (packages/node-server/test/data/)
• Added dependabot entry for gitsubmodule ecosystem to get automatic update PRs

Decisions

• The shared fixtures use a universal schema without variant or flagMetadata fields. The test code uses optional chaining (testCase.result.flagMetadata?.doLog) so missing fields are handled gracefully — exposure assertions are simply skipped for shared fixtures.
• The shared test-flag-that-does-not-exist.json uses DEFAULT reason (Go convention) but Node.js returns ERROR with FLAG_NOT_FOUND. Since the current test code on main does not assert reason, this difference is benign. The reason fix branch will add the appropriate override.
• Test count increased from ~100 to 251 due to additional evaluation cases in the shared fixtures (e.g., boolean-false-assignment, falsy-value-assignments, microsecond-date-flag).

Note: The submodule currently points at the leo.romanovsky/add-canonical-fixtures branch commit since fixtures have not been merged to main in ffe-system-test-data yet. Once that PR merges, update the submodule pointer with git submodule update --remote.