[typescript-operations] test: reproduce @oneOf input emitted as interface with declarationKind 'interface' (failing) (#10996)

[eddeee888]

Description

This PR adds a failing test that reproduces the bug reported in #10996, as a checkpoint before the fix. It adds only the test, not a fix.

With declarationKind: 'interface' and no importSchemaTypesFrom, typescript-operations emits a multi-field @oneOf input as an interface followed by a union. That isn't valid TypeScript. The shared base types visitor in visitor-plugin-common, which the typescript plugin uses, already forces a type alias in this case (#8586). The operations visitor has its own copy of the oneOf branch without that fallback (packages/plugins/typescript/operations/src/visitor.ts, InputObjectTypeDefinition).

The new test, generates @oneOf input as a type alias when declarationKind is interface in packages/plugins/typescript/operations/tests/ts-documents.standalone.input.spec.ts, expects the type alias. It currently fails on the inline snapshot:

-      export type FooInput =
+      export interface FooInput
         {   a: string; b?: never; }
-        |  { a?: never;   b: number; };
+        |  { a?: never;   b: number; }

The fix will be built directly on top of this commit. This PR doesn't need to merge, or go green, first.

Related #10996

Type of change

• Bug fix (non-breaking change which fixes an issue) — test only so far; the fix will follow

Screenshots/Sandbox (if appropriate/relevant):

Reporter's repro: https://stackblitz.com/github/jakubwawrzyczek/codegen-oneof-repro

How Has This Been Tested?

• Unit tests — the new test fails on purpose with the snapshot diff above, reproducing the bug (re-checked after rebasing onto the latest master)

🤖 Generated with Claude Code

https://claude.ai/code/session_01TsaydotU8AFHMT1P4MEfra

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