refactor(server): standardize generation error codes

[Graffioh]

Summary

• add shared, typed GenerateErrorCode values across all generation backends
• represent result state with a single std::optional<GenerateError> instead of separate boolean, enum, and string fields
• expose ok(), succeed(), and fail() helpers to keep success and failure states consistent
• serialize stable daemon error codes while retaining backend-specific details in logs

Why

Equivalent failures previously used inconsistent free-form strings such as prefill/prefill_failed, decode/decode_failed, and bad slot/invalid_snapshot_slot. The typed error model makes shared failures consistent and compiler-checked without losing architecture-specific diagnostics.

Compatibility

The daemon response shape remains err <code>, but recurrent legacy values are normalized to descriptive codes such as prefill_failed, decode_failed, context_overflow, and invalid_snapshot_slot. Backend-specific failures now use backend_specific; their detailed diagnostics remain available in server logs.

Validation

• compiled and ran a focused error-state and serialization check
• syntax-checked the daemon and LayerSplit backend
• added GenerateResult state-consistency coverage
• ran git diff --check

[cubic-dev-ai]

No issues found across 12 files

_{Re-trigger cubic}