This is a minor thing, but since S3CorsHandler seems independent from the provider, maybe we could further remove the dependency here, instead of introducing a dependency to both providers.
Suggestion (note: remove the call to BucketCorsIndex()):

introduce a Protocol here:

class BucketCorsIndex(Protocol):

    @property
    def cors(self) -> dict[str, CORSConfiguration]:
        raise NotImplementedError

    @property
    def buckets(self) -> set[str]:
        raise NotImplementedError


class S3CorsHandler(Handler):
    bucket_cors_index: BucketCorsIndex

    def __init__(self, bucket_cors_index: BucketCorsIndex):
        self.bucket_cors_index = bucket_cors_index
        ...

then, make sure to instantiate the provider-specific BucketCorsIndex in the provier:

since it's a protocol and it's just for type checking, there's also no need to let the implementations inherit from the protocol class.

nice 💯