Dev: Make getInsertID() more consistent with save()

### PHP Version

8.2, 8.3

### CodeIgniter4 Version

latest

### CodeIgniter4 Installation Method

Composer (using `codeigniter4/appstarter`)

### Which operating systems have you tested for this bug?

macOS, Linux

### Which server did you use?

fpm-fcgi

### Database

MaraiaDB

### What happened?

Not exactly a bug, but not really a FR either. See this forum thread: https://forum.codeigniter.com/showthread.php?tid=91800&pid=421782#pid421782

Essentially I'm offering to PR a change to `\System\Model::update()` that would set an instance's `$insertID` property the same way `\System\Model::insert()` does so that the behaviour of `save()` is more consistent.

Currently `\System\Model::getInsertID()` only returns a value if a `save()` does an insert. The logic needed to handle that works against the elegance provided by `save()`.

### Steps to Reproduce

Use `\System\Model::save()`

### Expected Output

Best case, `\System\Model::getInsertID()` will return the "affected ID" after `\System\Model::save()` regardless of which action occurred.

### Anything else?

I'm stoked contribute to core, but I didn't want to spend the time on a PR if there is no desire to change this behaviour. Please advise.

I can write the update to the docs too if I'm pointed in the right direction to do so.

Read the forum thread, then compare System Model's save(), insert(), update(), and getInsertID() behavior. Confirm the intended affected-ID semantics before changing anything; done means save() exposes the expected ID consistently for both insert and update paths, with coverage for each path.

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