Add retryability and expiration metadata to FFI error types

[chavic]

Directory/OHTTP/Session errors don’t expose “retryable/fatal” or expiration metadata. Clients cannot implement safe retry/backoff or decide when to abandon a session.

Todo:

• Add retryability flags to FFI error variants where applicable.
• Surface expiration timestamps/TTL when the session is already expired.
• Update sender/receiver tests to assert the retry/fatal signal.
• Add integration tests that simulate expired sessions and assert metadata.

[spacebear21]

Closing per #1264 (comment)