fix(types): fix infix types

[tharropoulos]

Change Summary

• Thread Infix through index.d.ts so TypesenseInstantsearchAdapterOptions and TypesenseInstantsearchAdapter preserve the upstream SearchParamsWithPreset<T, Infix> typing for infix.
• Add tsd tests in index.test-d.ts for constructor usage, including accepted valid infix literals and rejected invalid ones.
• This works by allowing TypeScript to infer the infix string literal at the adapter constructor boundary and apply the existing Typesense OperationMode validation.

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