fix(execution): settle permanent preparation rejections

[SaladDay]

A Runtime can definitively reject execution_prepare as unsupported while Core keeps retrying the same pending input until its five-minute deadline. Settle explicit configuration rejections through the existing runtime_preparation_failed path so the Session reports a safe failure and accepts corrected input without creating a Turn.

Only invalid_configuration, unsupported_configuration and unsupported_preparation for the current execution_prepare request with no Run are terminal here. Capacity, resource pressure, unconfirmed cleanup, disconnects and unknown responses retain their existing retry/deadline behavior. The existing failure settlement checks expiration after acquiring the Session lock, before applying failure to a still-pending input. Admission and Start handling, protocol/schema, Harness adapters and MCP preparation are unchanged.

Validation:

• Final-commit targeted tests passed on an isolated remote PostgreSQL database (36.0 s), including deadline evaluation after a blocked Session lock: safe failure and subsequent input, transient rejection retries with the original deadline, cancellation/newer-input and stale-request protection, existing preparation/disconnection/Start settlement and queued capacity behavior.
• Canonical Environment documentation and generated guide updated; git diff --check passed.
• A fresh independent blind review of the full final diff found no remaining issues.
• Final commit 5d7124b060e1fe395ab1569d024c58fca47ddac4: complete make check and daemon build, official-client acceptance, and Linux, macOS and Windows native checks all passed.

This is an immediate failure-reporting fix for a bounded retry, not a claim of infinite hanging. No deployment or stored-data changes; no release is published by this PR.

The remote make check attempt and a supplementary test attempt overlapped on their shared test database; both encountered the exclusive execution-owner lock. The targeted rerun on a newly created isolated database passed. The clean GitHub CI run is the full-gate acceptance source; the contended remote run is not a passing result.