feat(server): conditional BaseContext types via typed() factory (#2004)

[viniciusdacal]

Summary

• BaseContext is now generic over ContextFeatures — auth/tenancy fields only appear on ctx when the app has those features configured
• typed(auth) factory returns narrowed entity() and service() factories where hook ctx parameters reflect the configured features
• Zero breaking changes — BaseContext without a type parameter defaults to FullFeatures (all fields present), so all existing code compiles unchanged

Public API Changes

New types

• ContextFeatures, FullFeatures, NoFeatures — feature flag types
• AuthContext, TenantContext, MultiLevelTenantContext — mixin interfaces (from @vertz/server/entity)
• TypedFactories<F> — typed entity/service factory interface
• InferFeatures<TAuth> — derives features from auth config
• InferServerFeatures<TConfig>, InferServerContext<TConfig> — derives features from server config

New function

• typed(auth?) — returns { entity, service } with narrowed context types

Changed signatures (backwards compatible)

• BaseContext<TFeatures = FullFeatures> — now generic (was plain interface)
• EntityConfig<..., TFeatures = FullFeatures> — new 4th type parameter
• ServiceConfig<..., TFeatures = FullFeatures> — new 3rd type parameter
• entity() / service() — accept TFeatures type parameter
• defineAuth<T extends AuthConfig>(config: T): T — now generic (preserves literal types)
• TenantConfig.multiLevel?: boolean — new optional field

Design doc

• plans/2004-base-context-conditional-types.md

Test plan

• Type-level tests for all 4 feature tiers (NoFeatures, AuthOnly, AuthTenant, FullFeatures)
• EntityConfig hook ctx narrowing tests
• ServiceConfig handler ctx narrowing tests
• typed() factory type tests
• InferFeatures + InferServerFeatures tests
• defineAuth generic preservation tests
• @vertz/agents BaseContextLike structural compatibility test
• Backwards compatibility: BaseContext defaults to FullFeatures
• EntityContext assignable to BaseContext
• Runtime test for defineAuth identity behavior

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