#19343: Refactor chat state management: reset chat messages and queue on session change, and ensure chat history loading respects session key

Fix Control UI session isolation bug causing chat history to leak between dropdown sessions. Fixes #19187 <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR fixes a session isolation bug in the Control UI where chat history would leak between sessions when switching via the dropdown or navigation tabs. The fix has two parts: (1) `loadChatHistory` now snapshots the `sessionKey` before the async request and guards all state writes (including `chatLoading` cleanup in `finally`) against session mismatches, and (2) session-switch paths in `renderChatControls` and `resetChatStateForSessionSwitch` now eagerly clear `chatMessages` and `chatQueue` so stale data is immediately wiped before the new history loads. - `controllers/chat.ts`: Added session key snapshot and post-await session checks in `loadChatHistory` to prevent stale responses from overwriting the current session's state; the `finally` block now conditionally clears `chatLoading` only when the session key still matches. - `app-render.helpers.ts`: `resetChatStateForSessionSwitch` now clears `chatMessages` and `chatQueue`; the tab-click handler now calls `loadChatHistory` after resetting state; the session dropdown handler also clears `chatMessages` and `chatQueue`. - `app-render.ts`: The chat view's `onSessionKeyChange` callback now clears `chatMessages` on session change (it already cleared `chatQueue`). - Import reordering in all three files follows Oxfmt's type-first import ordering conventions. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge — it addresses a real session-isolation regression with targeted, well-reasoned changes and no regressions introduced. - The core `loadChatHistory` guard logic is correct and handles multi-switch races properly. The eager state clearing across all session-switch paths is consistent. No new APIs are introduced and the changes are confined to UI state management. Minor pre-existing inconsistency remains in the overview tab's `onSessionKeyChange` handler (which still doesn't clear `chatMessages`/`chatQueue`), but that handler doesn't directly trigger `loadChatHistory` and was already that way before this PR. - No files require special attention; the changes are focused and correct. _{Last reviewed commit: c7aed1a} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->