fix: Update DataSystemConfig to accept list of synchronizers

[keelerm84]

Note

Medium Risk
Touches core data-source failover logic in FDv2 and changes a public/experimental config shape, which could affect initialization and fallback behavior across environments.

Overview
Updates DataSystemConfig and ConfigBuilder to replace the primary_synchronizer/secondary_synchronizer model with an ordered list of synchronizers, validating at least one synchronizer when set and updating contract-test config parsing accordingly.

Refactors FDv2 synchronizer execution to iterate/fallback across N synchronizers (including removal of permanently failing sources, index-based recovery to the first synchronizer, and an explicit directive to switch to the FDv1 fallback synchronizer), and updates/extends unit tests to cover the new API and behaviors. CI contract tests are also bumped to v3 v3.0.0-alpha.3 and YAML quoting is normalized.

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[jsonbailey]

I added a suggestion but don't consider it a blocker.

[jsonbailey]

In Ruby I ended up collapsing this down to a single result, essentially adding the remove sync and fallback as options of the Conditional Directive. From my understanding they are all independent of each other. If you have fallback, you don't care about the other two, if you have remove sync, you don't care about the conditional directive, so there isn't a need to have all three returned.