fix(exec): fail on missing explicit resume targets

[middleDuckAi]

Summary

• stop codex exec resume <id> from silently creating a new thread when the requested target is missing
• keep resume --last behavior unchanged when there is no recorded thread to resume
• add both unit-level and end-to-end regression coverage for missing explicit resume targets

Why

Issues openai#15538 and openai#15539 both come from the same fallback: when an explicit resume target cannot be resolved locally, codex exec resume was falling through to thread/start and creating a new thread.

This change keeps the fallback for resume --last, but makes explicit resume <id> fail clearly with Session not found: <id>.

Testing

• cargo fmt --all
• cargo test -p codex-exec resolve_resume_target
• cargo test -p codex-exec exec_resume_by_missing_id_fails_without_starting_new_session
• cargo test -p codex-exec exec_resume_by_ephemeral_id_fails_without_starting_new_session

Notes

• This change is intentionally scoped to explicit resume targets.
• Missing explicit ids now fail instead of silently starting a fresh thread.