fix(collection): return the partial update schema from Collection.update

[cpruijsen]

Collection.update() in src/Typesense/Collection.ts now returns Promise<CollectionUpdateSchema>, which matches the partial PATCH response.
The previous CollectionSchema return type promised full collection properties that the response does not contain.
Callers that need name or num_documents can use retrieve().

CollectionUpdateSchema covers field-drop entries and metadata-only responses and matches the OpenAPI response definition.
Runtime request behavior is unchanged.

The regression tests in test/collection-update.type.test.ts require the partial return type and reject the full collection type.
Both fail against the original source and pass with this change.

Fixes #150.

[tharropoulos]

Thank you for the contribution! just a small nitpick about testing scope.

[tharropoulos]

instead of a type test here, could we add a type test and runtime validation under the existing Collection.spec.ts tests?

[cpruijsen]

Moved the test into the existing .update test in Collection.spec.ts. It now has a type assertion on the return value plus runtime checks on the response, and the standalone type test file is removed.

[tharropoulos]

Thank you for your contribution!