fix(site/src/pages/AgentsPage): disallow queued message edits - #28265

Closes CODAGT-230.

Selecting "Edit queued message" in Agent Chat did not stop the queued message from sending: edit mode was purely client-side state, so at turn end the backend promoted the queued message and sent it anyway, leaving the UI stuck in a stale "Editing queued message" state with the send button showing "Save".

This removes queued-message editing entirely instead of adding backend locking. Queued messages can still be promoted ("Send now") and deleted ("Remove from queue"); editing already-sent history messages is unchanged.

Changes

• Remove the Edit pencil from queued message rows (QueuedMessagesList), the "Editing queued message" banner and "Save" button label (AgentChatInput), and the queue-edit state, handlers, and prop plumbing (AgentChatPage, AgentChatPageView, ChatPageContent).
• Remove the queued-edit save flow that deleted the original queued row and re-queued a new message, plus the now-unused rawText/fileBlocks fields of getQueuedMessageInfo.
• Drop the redundant isEditingHistoryMessage prop on ChatPageInput, which had become identical to isEditing, and stop discarding the delete/promote promises so QueuedMessagesList busy state works again.
• Replace the queued-edit stories and tests with an ActionsExcludeEdit regression story asserting queued rows expose "Send now" and "Remove from queue" but no "Edit" button.

Testing

• pnpm -C site check, pnpm -C site lint:types.
• Unit: AgentChatPage.test.ts, QueuedMessagesList.test.ts (92 tests). Storybook: all four touched story files (102 tests). A pre-existing unhandled xterm error in AgentChatPageView.stories.tsx reproduces identically at the merge base.
• Remote dogfood UAT passed on a dev.coder.com workspace with a real model: no edit affordance on queued rows, queued delete and turn-end promotion still work, history-message editing and Escape behavior unaffected.

Mux acted on behalf of Mike for this pull request.