refactor(gfn): decompose AuthService responsibilities

[zortos293]

Reduce AuthService to a stable orchestration façade by moving its stateful responsibilities into focused GFN auth owners.

• Separate persisted account state, account transitions, provider discovery, subscription/VPC caches, and session-validity refresh coordination.
• Preserve legacy session restoration, provider normalization, account switching, token refresh, and cache behavior.
• Add focused coverage for persisted compatibility, account rollback/removal, and refresh-window decisions.

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