test(site/src/pages/AgentsPage): remove assertion-only story play functions - #29017

Storybook is visual regression only in this repository: the storybook CI job runs pixel-storybook, which loads each story, runs its play function, waits for the DOM to settle, and screenshots it. A play function that only asserts visible state duplicates what the screenshot already captures, and Pixel fails the capture only when a play throws.

This removes the play functions whose bodies are purely queries and assertions on visible state across the AgentsPage scope: 320 stories in 44 files.

What is intentionally kept:

• Every play that drives state the screenshot needs: menu opens, expand clicks, hovers, typing, and forced-visibility setup.
• The plays of pixel-excluded stories, so their behavior coverage is not lost.
• Stories whose play was their only property now render as plain visual states.

Coverage impact of the removal was checked one by one. The removed plays contained three assertions with no visual counterpart:

• ProviderRequiresUserApiKey's Settings link href is asserted identically in ModelSelectorHelp.test.tsx.
• AdvisorTool.Running's aria-expanded="true" is asserted by the surviving SuccessfulAdvice and LongAdviceLongQuestion stories.
• HiddenAssistantPlaceholderDoesNotRender's row-suppression behavior is covered by messageHelpers.test.ts "hides assistant messages whose execute tool renders nothing".

None of these removed plays asserted callbacks, storage, or real browser behavior that Pixel cannot see; nothing non-visual was lost, so no conversion was needed. Per the FE1 contract in #29016, what the DOM renders belongs to the screenshot, and tests assert the non-visual outcome of an interaction.

Generated by Coder Agents on behalf of @DanielleMaywood.