DefaultCryptoMaterialsManager should raise an error if signer key is present in encryption context

Because `DefaultCryptoMaterialsManager` adds a value to the encryption context if a signed algorithm suite is used, we need to throw an error if the requested encryption context already contains that key.

https://github.com/aws/aws-encryption-sdk-java/blob/eb8702e82759cca53eddc3b5f8423e74ec31aff1/src/main/java/com/amazonaws/encryptionsdk/DefaultCryptoMaterialsManager.java#L55-L58

Start by locating DefaultCryptoMaterialsManager in the Python repository and compare its signed-algorithm encryption-context handling with the cited Java implementation at DefaultCryptoMaterialsManager.java#L55-L58. Confirm the expected behavior for a context that already contains the signer-key entry, then add or update coverage so the operation raises an error in that case.

Written by the indexing model from the issue text.