Bug: numeric settings editor saves overflow values as null

### What happened?

The numeric settings editor accepts non-finite numeric values.

For example:

```ts
parseEditedValue('number', '1e309')
```

returns `Infinity` because the parser only rejects `Number.isNaN()` results.

The Settings dialog treats this as a valid value and passes it to `setSetting`. When the settings serializer writes the value to JSON, it becomes:

```json
{
"model": {
"compressionThreshold": null
}
}
```

This means an input accepted by the Settings dialog is silently changed into an invalid numeric configuration value.

I added this focused regression test locally:

```ts
expect(parseEditedValue('number', '1e309')).toBeNull();
```

I ran:

```console
npx --yes --package=node@20.19.0 node ./node_modules/vitest/vitest.mjs run src/utils/settingsUtils.test.ts --root packages/cli
```

Result:

```text
Test Files 1 failed (1)
Tests 1 failed | 49 passed (50)

Expected: null
Received: Infinity
```

### What did you expect to happen?

The numeric settings parser should reject non-finite values such as Infinity, -Infinity, and overflow values such as 1e309.

The Settings dialog should leave the existing setting unchanged instead of saving null.

### Client information

Client Information

Tested from the latest source checkout.

CLI Version 0.60.0-nightly.20260901.g0bd1d4397
Git Commit 85aca163f6c73ac6ce380b5447359146b8adcae4
Node.js 20.19.0
OS macOS 26.5.2 arm64
Sandbox no sandbox
Auth Method not applicable

### Login information

Not authentication-related. The behavior is in the local settings parser and serializer.

### Anything else we need to know?

Relevant code:

- packages/cli/src/utils/settingsUtils.ts: parseEditedValue
- packages/cli/src/ui/components/SettingsDialog.tsx: handleEditCommit
- packages/cli/src/utils/commentJson.ts: settings serialization

I searched existing open and closed issues and pull requests using Infinity, non-finite, 1e309, numeric input, and settings validation terms. I did not find a matching report or implementation.

I am willing to implement a small fix with regression tests if maintainers consider this suitable for a community contribution.

Start with parseEditedValue in packages/cli/src/utils/settingsUtils.ts and the focused cases in src/utils/settingsUtils.test.ts under packages/cli. Check how SettingsDialog.tsx handleEditCommit uses the parsed result and review commentJson.ts for the serialization path; done means non-finite numeric input is rejected and the existing setting is not replaced by null.

Written by the indexing model from the issue text.