Audit `SessionError`, `CreateRequestError`, and `EncapsulationError` internal variants

[spacebear21]

• process_response methods return inconsistent error types when POSTing vs. GETting, specifically: EncapsulationError vs. ResponseError for send and SessionError vs. Error for receive.
• There are four duplicated variants between InternalEncapsulationError and InternalSessionError.
  • Also, EncapsulationError is a misnomer since it afaict it's exclusively raised when decapsulating responses.
  • The Hpke variant has nothing to do with en/decapsulation.

Originally posted by @spacebear21 in https://github.com/payjoin/rust-payjoin/issues/660#issuecomment-2814308584

There are other variants that appear duplicated between InternalCreateRequestError and InternalSessionError (namely Expired, Hpke, OhttpEncapsulation and potentially Url/ParseUrl).