fix(acp): improve process cleanup and add type annotations

[carlos-algms]

ACP processes were not being closed, leaving behind orphan zombie processes.

In my machine, I had more than 30 zombie processes.

The previous transport_self.process:close() only closes the communication between Neovim and the spawned process, but it doesn't kill the process.

Now, it sends 15 to start a graceful exit flow, and if it gets stuck, it sends a force termination with 9.

Changes

• Add proper process termination with SIGTERM (15), then SIGKILL (9)
• Check if the process is closing before cleanup to prevent errors
• Add type annotations for ACPTransportInstance
• Use vim.uv with fallback to vim.loop for compatibility

How to test

Before, original behaviour:

1. open neovim
2. open avante
3. write any prompt and submit
4. exit with :qa
5. run ps aux | grep <your acp provider name, mine is claude-code-acp>
6. you should see at least 1 orphan process

Using these changes

After step 4, exit with :qa and then run step 5.
You should see 0 orphan processes.