Bug: list_background_processes prints (Exit Code: null) for signal-killed processes

### What happened?

`list_background_processes` prints `(Exit Code: null)` when a background process was killed by a signal (or backgrounded with `exitCode: null`).

`BackgroundProcess.exitCode` is `number | null | undefined` (`packages/core/src/services/shellExecutionService.ts`). Signal / backgrounded paths set `exitCode: null`.

The formatter only checks `!== undefined`:

```ts
// packages/core/src/tools/shellBackgroundTools.ts
p.exitCode !== undefined ? ` (Exit Code: ${p.exitCode})` : ''

```
null !== undefined is true, so the model sees:

- [PID 1234] EXITED: \sleep 60` (Exit Code: null)`

The exit-code clause should be omitted when there is no numeric code. Sibling checks in shell.ts already use !== null (e.g. around the display path). #29043 is the same class of bug in shell.ts sandbox-denial logic — this is the list-background-processes display path.

Repro:

Background a process, then kill it with a signal so exitCode === null.
Call list_background_processes.
Output contains (Exit Code: null).
Verified on current main.

### What did you expect to happen?

If exitCode is null or undefined, omit the exit-code clause.

If exitCode is a number, print (Exit Code: N).

If a signal is present, print (Signal: …) only.

### Client information

Reproduced from source on current main. Platform: macOS. CLI Version: 0.59.0-nightly.20260825.g812f7a2bc

### Login information

Not auth-related. Logic bug in list_background_processes formatting

### Anything else we need to know?

Suggested fix: change the guard to typeof p.exitCode === 'number'.

Add a unit test where exitCode is null and assert llmContent does not contain (Exit Code: null).

Related but not a duplicate of #29043 (different file and symptom).

I'd like to send a focused PR with tests after this is labeled help wanted.

Start in packages/core/src/tools/shellBackgroundTools.ts and inspect the exit-code formatter, then compare the sibling checks in shell.ts. Add a focused unit test covering exitCode null and verify numeric exit codes still appear. Done means list_background_processes omits the exit-code clause for null or undefined, while showing it for numbers and showing signal information when present.

Written by the indexing model from the issue text.