Fix hydrateMessages silently dropping user messages

[cstns]

Found while building the onboarding surface, but this is shipped behaviour: hydrateMessages in the product-expert store rehydrates a saved transcript with a switch (true), which matches cases by strict equality. The isUserMessage predicate returned hasOwnProperty && message.query, which evaluates to the query string rather than a boolean, so it never strictly equals true and every user message is silently skipped. The AI branch only works because Array.isArray happens to return a real boolean.

In practice this means the editor to platform conversation handoff (wakeUpAssistant with shouldHydrateMessages) rebuilds the transcript with the assistant's answers but none of the user's messages.

Both predicates now return actual booleans, with a comment explaining why that matters, plus a regression spec that hydrates a mixed transcript and asserts the message sequence.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.77%. Comparing base (4d85e86) to head (95eb61a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8442   +/-   ##
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  Coverage   76.77%   76.77%           
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  Files         460      460           
  Lines       24703    24703           
  Branches     6573     6573           
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  Hits        18966    18966           
  Misses       5737     5737           

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