fix: wait_agent timeout for queued mailbox mail

[jif-oai]

Why

wait_agent can be called while mailbox mail is already pending. The previous implementation subscribed for future mailbox sequence changes and then waited for the next notification. If the mail was queued before that wait started, no new notification arrived, so the tool could sit until timeout_ms even though mail was ready to deliver.

What Changed

• Added Session::has_pending_mailbox_items() for checking pending mailbox mail through the session API.
• Updated multi_agents_v2::wait to return immediately when pending mailbox mail already exists before sleeping on a new mailbox sequence update.
• Reworked the regression coverage in multi_agents_tests.rs so already queued mailbox mail must wake wait_agent promptly.

Relevant code:

• wait_agent pending-mail check
• Session::has_pending_mailbox_items
• multi_agent_v2_wait_agent_returns_for_already_queued_mail

Verification

• cargo test -p codex-core multi_agent_v2_wait_agent_returns_for_already_queued_mail

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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