fix(mcp-server): Handle oneOf documents as operation input/output roots

[psp65]

What behavior changes?

McpService no longer fails (or silently degrades) when a shape carrying the smithy.mcp#oneOf trait is used directly as an operation input or output.

With a model like:

@oneOf(discriminator: "__type", members: [...])
document ShapeWithOneOf

operation GetShape {
    output: ShapeWithOneOf
}

operation ProcessShape {
    input: ProcessShapeInput // contains a member targeting ShapeWithOneOf
}

building the McpService previously had two order-dependent failures, because the per-service schema cache is shared across schema kinds:

• If a nested reference (ProcessShape) was processed first, the cache held a JsonOneOfSchema for the shape, and createJsonObjectSchema for the GetShape output root then threw ClassCastException: JsonOneOfSchema cannot be cast to JsonObjectSchema — failing construction of the entire service (every tool), not just the offending operation.
• If the root reference was processed first, createJsonObjectSchema treated the document as a plain (member-less) shape and cached an empty JsonObjectSchema under the shape id, which nested references then reused — silently dropping all oneOf variants.

After this change both positions render correctly: the root position produces {"type": "object", "oneOf": [...]} (the MCP spec requires tool input/output schemas to be object-typed; the oneOf variants are all objects, so the constraints compose), and nested references keep the cached JsonOneOfSchema exactly as before.

Why is this change needed?

Bundled models (loaded through ModelBundles, which assembles with validation disabled) can and do contain document-typed operation outputs: tooling that converts service models for MCP represents polymorphic type hierarchies as @oneOf documents, and a service with a polymorphic operation output currently cannot list tools at all. Observed in production as a deterministic ClassCastException at McpService.createJsonObjectSchema for every such service.

How was this validated?

• Two new regression tests in McpServerTest, one per operation processing order (operations are processed in sorted order, so each order is pinned by operation naming):
  • testOneOfDocumentAsOperationOutputRoot — root processed first: asserts the object-position rendering of the @oneOf document root AND that a nested reference to the same shape keeps its variants (the silent cache-pollution mode).
  • testOneOfDocumentAsOperationOutputRootWithCachedSchema — nested reference processed first, so the root request hits a cached JsonOneOfSchema (the ClassCastException mode).
  • Both test models are assembled with disableValidation() to mirror ModelBundles.
• Verified both tests fail on main without the fix and pass with it.
• ./gradlew :mcp:mcp-server:test passes.

What should reviewers focus on?

• McpService#asJsonObjectSchema: the re-shaping of a JsonOneOfSchema into an object-typed schema, and the new @oneOf guard in createJsonObjectSchema that routes trait-carrying shapes through createJsonOneOfSchema so the cache keeps the full oneOf schema for other references.
• mcp-schemas/model/main.smithy: JsonObjectSchema gains an optional oneOf member so the object-typed root can carry the variants. The converted root preserves the type: "object", oneOf, and description a JsonOneOfSchema would serialize, plus the $schema annotation that all object-typed roots already carry.
• The new guard in createJsonObjectSchema is scoped to ShapeType.DOCUMENT (the trait's selector), so a non-document shape incorrectly carrying the trait in an unvalidated model keeps its existing rendering — matching what runtime input/output adaptation recognizes.

Additional Links

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