fix(site/src/pages/AgentsPage): treat interrupting chats as busy in the composer - #28209

…he composer

The composer derived isStreaming from hasStreamState or a running chat
status, excluding interrupting. Once an interrupt is requested the
stream state is gone, so a chat still finalizing an interruption
rendered the idle composer: no Stop button, the normal Send button,
and no busy indication. That made the interrupting state visually
indistinguishable from waiting.

Include interrupting in the composer's streaming check and add
Storybook coverage for the I1 state (interrupting with a queued
message) alongside a running-state control.

…rupting

The interrupt POST resolves as soon as the request lands, but the
worker may finalize the interruption for several seconds, and the
backend rejects a second interrupt with 409. Gate the composer's stop
action on the interrupting status in addition to the mutation's
pending flag so neither the Stop button nor Escape re-fires an
interrupt that the close state machine (I0/I1) would reject.

Also trims the interrupting story comment to the non-obvious invariant
only.

…story

The RunningShowsBusyComposer header comment restated what the story
code and assertions already say; FE4 prohibits that narration.

…ueued-message fixture

The interrupting story now focuses the composer and sends Escape,
asserting the interrupt callback is not invoked while finalization is
still pending. The queued message is built from MockChatQueuedMessage
instead of an inline literal so fixture drift type-checks.

isActiveChatStatus is intentionally not reused for the composer's
busy check: it means active for store synchronization, a broader
contract than the composer needs, and conflating the two could show a
Stop button for a future busy status where interrupting is not legal.

… role

Prefer getByRole("textbox", { name: "Chat message" }) over the test ID
when focusing the editor in the interrupting story, so a regression
that drops the accessible name fails instead of passing on a stale
test ID.

…eyboard users

While an interrupt is finalizing, the composer's Stop button is
disabled and skipped in tab order, leaving keyboard and screen-reader
users with no indication that the interruption is still in progress.
Wrap the button in a tooltip that reads "Interrupting..." while
pending, and add an sr-only live region with role=status announcing
the same. The interrupting story now asserts the live region text.

…during finalization

While an interrupt finalizes, the interrupted turn's flushed stream
state made the transcript render "Thinking" as if the agent were still
producing output. deriveLiveStatus now takes chatStatus and the new
interrupting phase outranks stream leftovers, so the activity slot
reads "Interrupting" with a pause icon (matching the sidebar's
interrupting status icon) instead. The interrupting story asserts the
transcript label and the absence of Thinking.

….each typing

Removes a comment that restated the story assertions, and replaces the
as-const cast in the deriveLiveStatus case table with a typed cases
array plus satisfies annotations on the expected status fixtures.