Avoid hot probes in operator snapshots

[yvette-carlisle]

Summary

• keep serve/control-plane operator snapshots on snapshot-only account activity reads
• avoid full runtime refreshes for shared-claim and retry-budget status checks
• add regression tests for account snapshots and persistent hot-path reads

Verification

• cargo test -p decodex idle_operator_status_snapshot_includes_configured_codex_accounts --all-features
• cargo test -p decodex account_activity_snapshot_uses_configured_records_without_usage_probe --all-features
• cargo test -p decodex persistent_ --all-features
• cargo make check