No test coverage for the chat-history turn-admission predicate

[tabmail-kmyi]

What is uncovered

DynamicIslandChatButton.messagesForSession filters which chat turns render in session history.
Its predicate now admits only turn.type == "normal"; the previously-admitted "task_result"
branch was removed when scheduled-task execution was taken out of the app.

That predicate has no test coverage in either direction:

• no test asserts a "normal" turn renders,
• no test asserts a non-"normal" turn does not.

Why it was left uncovered

messagesForSession is a private func on a View struct carrying ~20 property wrappers, so it
cannot be reached from a test as written. Covering it requires extracting the predicate into a
pure, testable helper — a production change wider than the deletion that exposed the gap, so it
was deliberately not bundled into that change.

Suggested fix

Extract the turn-admission predicate into a pure function over the turn's type (a free function or
a static member — no View instance, no property wrappers), have messagesForSession call it, and
test it directly.

Per the repo's refactoring rule, write the characterisation tests first against current
behaviour, verify they pass, then extract, then verify the same tests still pass.

Acceptance

• A test proves a "normal" turn is admitted.
• A test proves an unknown/other turn type is rejected, and that test fails if the predicate is
  widened back to admit it (red-first, so it pins the invariant rather than the current spelling).
• The invariant under test is only explicitly-admitted turn types render, not a fact about any
  one type name.