Send Expert chat messages one at a time

[andypalmi]

Every message to the Expert is now queued so only one is in flight per session. Before, the instance ready/failed notice was published straight away, so if the instance came up while a reply was still streaming, two turns ran on the same session and each overwrote the other's chat history. That showed up as two separate chats.

Chat turns, opening a conversation, approval resumes and the instance notices all go through one queue in the Expert store. A job is finished when its reply arrives, or when the turn is stopped or errors.

• Stop only ends the turn in flight. The next queued message goes out after it, and can be stopped in turn.
• Start Over drops the queue and no longer stays waiting on the old session's reply.
• The "workspace ready" card still shows straight away, only the message to the Expert waits for its turn.
• The composer stays disabled while a reply is pending, so queued messages are the system notices only. Sending several user messages at once needs the Expert to accept a list, so that is left for later.

Tests cover a notice arriving mid-turn, order, stop letting the next one through, and Start Over clearing the queue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.54%. Comparing base (21c5dca) to head (e228dc5).

Additional details and impacted files

@@                     Coverage Diff                     @@
##           feat/expert-build-tracking    #8876   +/-   ##
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  Coverage                       78.54%   78.54%           
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  Files                             475      475           
  Lines                           25760    25760           
  Branches                         6857     6857           
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  Hits                            20234    20234           
  Misses                           5526     5526           

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