#22076: [Bug]: Preserve chat draft when aborting

## Summary Fixes #22063 When aborting a running chat stream, the UI previously cleared `host.chatMessage` in `handleAbortChat()`, which made users lose draft text typed during generation. ## What changed - Removed the unconditional draft reset from `ui/src/ui/app-chat.ts`: - Deleted `host.chatMessage = ""` from `handleAbortChat()`. - Added regression coverage in `ui/src/ui/app-chat.test.ts`: - Verifies the typed draft is preserved when `handleAbortChat()` is invoked while connected. - Verifies no abort request is sent when the UI is disconnected. ## Why this is correct - Aborting a chat run is server-side cancellation and does not require clearing user input. - Chat draft input now mirrors expected behavior from mainstream chat UX patterns: stop/abort does not erase in-progress text. - This is a tightly scoped behavior fix with no protocol/client payload changes and no API surface changes. ## Validation - Ran targeted UI test: - `cd ui && pnpm test -- src/ui/app-chat.test.ts` - Result: pass (2/2) ## Risk Assessment - Very low risk; only one assignment removal and direct behavioral guardrail. - No branching behavior changes outside abort path. - No security, persistence, or transport protocol changes. ## Confidence Score - 10/10 confidence - Patch is minimal and deterministic. - Regression test covers the reported draft-loss path and a disconnected no-op path. - No unrelated refactors or dependency changes. - Expected merge impact is limited to UX behavior in `ui/src/ui/app-chat.ts`. <!-- greptile_comment --> <h3>Greptile Summary</h3> Removed unconditional `chatMessage` reset from `handleAbortChat()` in `ui/src/ui/app-chat.ts:67`, preserving user's draft text when aborting a running chat stream. Added regression tests covering both the connected abort path (verifying draft preservation) and disconnected no-op path. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The change is a single-line deletion with clear UX improvement, comprehensive test coverage for the regression, and no impact on other code paths or API contracts - No files require special attention _{Last reviewed commit: f11f34a} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->