fix: run post-scan prompts after failed scans

[ianw-oai]

Problem

The post-scan prompt only runs after a complete scan. Failed and incomplete scans skip cost accounting and other follow-up work.

Change

Run the prompt after failed or incomplete scans. Keep the original scan error, report follow-up errors as warnings, and do not start another model turn after cancellation or a cost limit.

Tests

• 958 SDK tests passed before the update to current main.
• Six focused tests passed on current main.
• Type checks, formatting, and the package build passed.