error handling readOnly input's validity object and validationMessage

Nobody has claimed this yet.

### What is affected?

Component

### Description

``

Default step value for this is "1". Now if you set this **readOnly** input in JS document.getElementById("123").value = "0.5" you'll get following:
```
input.reportValidity() = true
input.checkValidity() = true
```
BUT
input.validity Object will have "stepMismatch: true"
and
input.validationMessage = ''.
This seems to be a problem for material-web internal validation logic. It doesn't like a property of the validity object to be 'true' and an empty validationMessage at the same time.

### Reproduction

create a number readOnly material web input and set its value to "0.5"

### Workaround

Ensure all the inputs would be theoretically valid, although validation shouldn't be triggered in this case.

### Is this a regression?

No or unsure. This never worked, or I haven't tried before.

### Affected versions

1.5.0

### Browser/OS/Node environment

Chrome, Windows

No source file or test is named. Start by reproducing the readOnly number input case in Chrome with value "0.5", then trace the material web input's validation handling and compare reportValidity(), checkValidity(), validity.stepMismatch, and validationMessage. Done means the component handles this combination consistently without breaking expected validation behavior.

Written by the indexing model from the issue text.