Formalize skipReason in ProviderAdapter return type

[mschilling]

Both GitHub and Azure DevOps adapters use { skipReason: string } as any / as Partial<GameAction> to signal that an event should be skipped. This works but bypasses type safety.

Proposal: Update the ProviderAdapter.mapEventToGameAction return type to:

type MapResult = Partial<GameAction> | { skipReason: string } | null;

And update GameActionService to check for the discriminated union properly.

Affected files:

• libs/server/integrations/src/lib/providers/interfaces/provider.interface.ts
• libs/server/integrations/src/lib/providers/github/github.adapter.ts (10+ occurrences)
• libs/server/integrations/src/lib/providers/azure-devops/adapter.ts (2 occurrences)
• libs/server/integrations/src/lib/services/game-action.service.ts

Ref: PR #293 review feedback