Change Proposal: Add an accessible error state to NumberInput for other components to react to

## What is being proposed?

As picked up by [Atlas #143](https://github.com/DiamondLightSource/atlas/pull/143#issuecomment-5207977415), the `NumberInput` currently shows user-entered invalid values, but does not commit them. As such, any form populated and used for further action may use outdated values.

## Why is this needed?

Current behaviour of components which act on inputs from `NumberInput` is unpredictable for the user, as invalid values are shown as true to a user, but not committed. This could be tracked by an accessible error state which other components can use to disable on an invalid entry.

## What will change?

Depending on implementation methods, this may change component interface, behaviour of committing an invalid value, etc.

## Interface changes (if any)

Potentially, but undetermined.

## Breaking change?

- [ ] Yes
- [ ] No

## Next steps
A maintainer will review this issue.
If accepted, it will be marked as `accepted` and a PR may then be opened.

Start by inspecting the NumberInput component and its current handling of invalid user-entered values. Define how an accessible error state should be exposed and how other components can react to it; the issue does not identify tests or a settled interface, so completion criteria require maintainer agreement.

Written by the indexing model from the issue text.