[codex] Preserve failed goal accounting flushes

[jif-oai]

What

• Preserve database accounting failures from the goal extension instead of collapsing them into None
• Warn with turn/tool context when a flush fails
• Keep stop/abort accounting snapshots alive when the final flush did not persist

Why

PR #23696 can finish and discard a turn snapshot after account_thread_goal_usage fails. That loses the final accumulated accounting state silently. This follow-up keeps that failure explicit and avoids deleting the local snapshot in the failing path.

Testing

• just fmt
• cargo test -p codex-goal-extension

[jif-oai]

@codex review

[chatgpt-codex-connector]

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