Job code AI assistant: Show statuses streamed after the text answer

[hanna-paasivirta]

Description

If Apollo streams a thinking status (e.g. "Writing code…") after the text answer while job code is still being generated, it will now be displayed below the answer using the same indicator style as the pre-text "Thinking…" status.

Closes #4832

Currently the order of events from Apollo when there are code changes is strictly stream status -> text -> stream status -> code. However, I tried to implement this in a way that will also work if we add more interleaved text/stream statuses.

Validation steps

1. Open a workflow step.
2. Ask the assistant (not the experimental global assistant) for code changes, or new code if generating from scratch. Ask for extensive changes requiring more than a couple of lines of changes.
3. Verify that you see:
   -> streaming statuses
   -> text explanation of what the assistant is going to do
   -> new: another streaming status
   -> code diffs are noted in the chat and displayed the code.

AI Usage

Please disclose whether you've used AI anywhere in this PR (it's cool, we just
want to know!):

• I have used Claude Code
• I have used another model
• I have not used AI

You can read more details in our
Responsible AI Policy

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review
  with Claude Code)
• I have implemented and tested all related authorization policies.
  (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review ✅

• S0 (project scoping): N/A — frontend-only change to AI assistant streaming status UI (MessageList.tsx, AIChannelRegistry.ts, createAIAssistantStore.ts); no queries or data access modified.
• S1 (authorization): N/A — no new web-layer entrypoints, LiveView events, or controller actions; only client-side store/component behavior for rendering streaming status.
• S2 (audit trail): N/A — no config-resource writes or Ecto.Multi changes; backend untouched.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.3%. Comparing base (f590caa) to head (c99883e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4833   +/-   ##
=====================================
  Coverage   90.3%   90.3%           
=====================================
  Files        444     444           
  Lines      22577   22577           
=====================================
+ Hits       20393   20394    +1     
+ Misses      2184    2183    -1     

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