Contextual REST companions

[sebaciv]

Adds contextual and custom-implicits REST companion APIs to udash-rest, together with their ScalaDoc, unit tests, and guide documentation.

New companions

Contextual APIs — let a server-side method implementation receive request-scoped context (e.g. the authenticated user) that never appears in the client-facing interface:

• WithCtx[Ctx, R] and the CtxTask[T] = WithCtx[Ctx, Task[T]] alias, with CtxTaskOps combinators and a CtxTaskCompanion factory; NoCtx marks the context-free client side.
• ContextualServerRestApis[Implicits, Ctx] — companions for server-only contextual APIs (ServerApiCompanion, ServerApiImplCompanion).
• ContextualServerAndClientRestApis[Implicits] — companions for contextual APIs shared between server and client (the client fixes the context to NoCtx).

Custom-implicits companions — packaged companion sets bound to a user-supplied implicits bundle, so custom serialization/schema is threaded into macro derivation without per-shape boilerplate:

• RestApisWithCustomImplicits[Implicits] — ApiCompanion / NoDocApiCompanion / ServerApiCompanion / ClientApiCompanion.
• ApiDataWithCustomImplicits[Implicits] — data-type companions: ApiDataCompanion, ApiDataCompanionWithDeps, ApiSealedCaseCompanion, ApiSealedSubHierarchyCompanion, and poly companions (PolyApiDataCompanion, PolyObjectApiDataCompanion, Poly2ApiDataCompanion).

Supporting helpers: RestNamedValueEnumCompanion, HasRestSchema, RestFlattenedStructure, CaseNameValidatingCodec.

Documentation (ScalaDoc)

• ScalaDoc for all of the above, with usage examples.
• Examples use DefaultRestImplicits (which already supports both Task and Future).

Tests

Unit tests for the new companions (run on JVM, cross-compile on JS):

• WithCtx / CtxTaskOps / CtxTaskCompanion combinators
• Contextual server-only and server+client (NoCtx) round-trips
• RestApisWithCustomImplicits round-trip + OpenAPI generation
• ApiDataWithCustomImplicits: sealed-case, sealed-subhierarchy, poly (1-arg / object / 2-arg) and WithDeps companions
• RestNamedValueEnumCompanion schema, CaseNameValidatingCodec discriminator validation, and RestFlattenedStructure.caseRestSchema

The tests deliberately use a type with no default serialization whose codec/schema is provided only through injected implicits — verifying that a custom implicits bundle is actually collected by the derivation macros.

Guide (rest.md)

• New "Contextual REST APIs" section (WithCtx / CtxTask, server Ctx vs client NoCtx).
• Documented RestApisWithCustomImplicits / ApiDataWithCustomImplicits in the custom-serialization section.
• Fixed a stale MonixRestImplicits example link.