[APP-4285] Send slash commands as follow-ups with active AI streams (#9243)

## Description

Running `/pr-comments` (or any other slash command that targets the currently-selected conversation) while an AI response stream is in-flight crashed in debug builds. `BlocklistAIController::send_request_input` hits its in-flight invariant and fires `safe_assert!(false, ...)` (panics in debug, returns `Err` in release).

The user-query path (`send_query`) avoids this because it pre-cancels any active stream on the target conversation with `CancellationReason::FollowUpSubmitted` before calling `send_request_input`. Slash commands bypass that path and dispatch directly via `SlashCommandRequest::send_request`, so the cancel never happens.

This PR makes `send_slash_command_request` mirror that cancel-and-resend: if the target conversation has an in-flight stream, cancel it before dispatching. All `SlashCommandRequest` variants are conceptually a fresh user turn (a follow-up), so this matches the semantics users already get from typing a follow-up message.

`send_queued_slash_command_request` is unchanged — it's only invoked from `Input::submit_queued_prompt` once the conversation is idle, so no pre-cancel is needed.

## Testing

Verified locally. Demo [here](https://www.loom.com/share/a9638ac9a53b48349ce21d03eaba516a)!

- Manually reproduced the crash on a dogfood debug build by triggering `/pr-comments` mid-stream; confirmed the panic at the `safe_assert!` in `BlocklistAIController::send_request_input`.
- After the fix: same repro cancels the in-flight turn and dispatches `/pr-comments` cleanly. Same behavior verified for `/skills` (InvokeSkill), `/compact` (Summarize), and `/create-environment`.

## Agent Mode

- [x] Warp Agent Mode - This PR was created via Warp's AI Agent Mode

## Changelog Entries for Stable

CHANGELOG-BUG-FIX: Fixed an issue where slash commands sent while an agent was still responding were silently dropped. Now, slash commands like `/pr-comments` run as follow-ups, just like typed messages.

---

Run: https://staging.warp.dev/conversation/aeecfa2d-1456-440b-9961-c27295d82531
Plan: https://staging.warp.dev/drive/notebook/Ryj2w6xDSqDwNJYhpwwI0Vly

app/src/ai/blocklist/controller/slash_command.rs

@@ -168,7 +168,7 @@ impl SlashCommandRequest {
         }
     }
 
-    fn conversation_id(
+    pub(super) fn conversation_id(
         &self,
         controller: &BlocklistAIController,
         app: &AppContext,