fix: fall back to provider state when managed dolt is unpublished

[azanar]

Summary

This teaches GC-managed Dolt target resolution to fall back to valid provider runtime state when the published managed-Dolt state file is still missing.

What changed

• add currentResolvableManagedDoltPort() to prefer published runtime state but fall back to valid provider runtime state
• allow resolvedRuntimeCityDoltTarget() to use that provider-state port during recovery when the canonical managed target is still unpublished
• cover the gap with a focused regression test

Why

There is a startup window where managed Dolt is already running and provider state is valid, but the published runtime state file is not present yet. During that window, GC-managed startup paths could still resolve a local managed target without a port and fall through to 127.0.0.1:0 failures.

This keeps the target resolution aligned with the real managed runtime state during that gap.

Tests

• go test ./cmd/gc -run 'TestBdRuntimeEnv(DoesNotUseStalePortFileWithoutManagedRuntimeState|UsesValidProviderStateWhenPublishedStateIsMissing)' -count=1

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