fix(ahp): preserve tool results when observers fail

[danielkov]

Summary

Preserve completed tool results when an observe-only tool.after hook fails during preparation or dispatch. Retain attachment ownership through the SDK's typed boundary and surface sanitized dispatch diagnostics through the existing session hook-message channel.

Motivation

Kit discarded owned attachment sources when converting tool.after inputs to raw events. It also converted top-level observer preparation failures into CustomPolicyDenied, withholding otherwise successful tool results.

Technical details

Use the typed tool_after SDK entry point so binary attachment sources survive projection and participate in SDK preparation. Observer-only dispatch failures retain the native outcome, including output and execution metadata.

Any registered tool.after interceptor conservatively retains fail-closed behavior for dispatch failures, including mixed observer/interceptor configurations and payload-filtered interceptors. Native failures and cancellation remain intact.

Diagnostics expose the SDK classification rather than arbitrary error strings that may contain output or secrets. This change does not raise attachment limits or alter the SDK's attachment-admission ordering.

[kit-code-agent]

No issues found. The changes look good to merge.

_{Took 2 min 34 sec}

[kit-code-agent]

No issues found. The changes look good to merge.

_{Took 2 min 8 sec}