fix: resolve ACP v1 conformance failures found by acp-tck

[EugeneTheDev]

Runs the acp-tck ACP v1 conformance suite against the adapter and fixes every MANDATORY and CAPABILITY failure it reports; the verdict goes from NOT CONFORMANT to CONFORMANT.

• session/resume, session/load and session/delete failed with "no rollout found for thread id" on a session that was created but never prompted, because Codex materializes a thread's rollout lazily on its first user message. Resume/load now fall back to thread/read, which answers for a live unmaterialized thread; delete treats "no persisted thread under this id" as success, which also makes deleting an unknown or unparseable session id a silent no-op as the spec asks.
• session/load could be followed by a session_info_update from a title generation still running from an earlier turn. Load now waits for that generation to settle before answering.
• A session/cancel that lands before Codex registers the turn as interruptible was dropped, and the turn answered end_turn instead of cancelled. turn/interrupt is now retried with a short backoff while the prompt is still in flight.

Left as-is, ADVISORY-only: the remaining ACP-SCHEMA-002 finding flags PromptResponse.usage and NewSessionResponse.models as non-_meta root fields. usage is an SDK-defined field, and models is an intentional backwards-compat shim for pre-configOptions clients whose removal would need a deliberate major-version deprecation rather than a conformance patch. Since the requirement is ADVISORY, this doesn't affect the CONFORMANT verdict.