WordPress platform: question answers strand the original run in input_required #778

Problem

The question/actions/upload round trip in the WordPress platform plugin (#773) doesn't line up with what the WP side produces.

• Nothing emits chat_input events. WpCodingAgents_Runtime_Chat only queues chat_message and chat_cancel, so handleInput() and the agents/claim-chat-run-input drain in wordpress-platform.mjs are unreachable in production.
• In practice the answer arrives as a new agents/chat turn. That turn has a new run_id and claim token. handleTurn() swaps currentRun to the new run and delivers the answer. The original run is left in input_required forever: nothing finalizes it, and clients polling agents/get-chat-run for it see a permanently waiting run.

Target state

Pick one canonical answer path and make both sides agree.

• Preferred: answers to an input_required run go through the agents-api queued-input contract that agents/claim-chat-run-input drains (WP_Agent_Chat_Run_Control::queue_message / can_queue_message). The WP side then wakes the runtime with a chat_input event for the same run, so a single run spans question → answer → completion.
• If a new turn is the answer: the plugin finalizes or supersedes the previous run explicitly (status plus a linking event) before adopting the new one.

Check how agents/chat currently treats a message sent to a session whose run is waiting (agents-api #550, waiting-for status) before choosing.

Deliverable

A committed change with tests covering:

• A question answered from a WP client resumes the session.
• Every run ends in a terminal status.
• chat_input (if kept) has a producer.

CHANGELOG and VERSION are release-owned and stay as they are.