fix(keyservice): return error for unsupported master key types

[santosh7676]

Summary

• Replace panic in keyservice.KeyFromMasterKey when encountering an unknown keys.MasterKey type with explicit error handling.
• Add KeyFromMasterKeyOrError and ErrUnsupportedMasterKeyType for callers that need to handle unsupported key types safely.
• Update core encrypt/decrypt paths in sops.go to propagate conversion errors instead of crashing the process.
• Add regression tests to ensure unsupported key types return an error and do not panic.

KeyFromMasterKey previously panicked on unknown master key types. That can crash SOPS during encrypt/decrypt operations instead of returning a recoverable error. This change makes failure behavior consistent with the rest of the key handling flow.

Behavior change

• Before: unsupported MasterKey type -> process panic.
• After: unsupported MasterKey type -> returned error (ErrUnsupportedMasterKeyType) and normal error propagation in encrypt/decrypt paths.