fix: preserve value on unparsable input

[ugur-vaadin]

Description

This PR updates the validation logic for DatePicker and TimePicker so that the value is preserved on setting unparsable input in a field with a valid value.

Fixes #8209

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/contributing/overview
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

[sonarqubecloud]

Quality Gate failed

Failed conditions
11.0% Duplication on New Code (required ≤ 10%)

See analysis details on SonarQube Cloud

[vursen]

The reported issue seems to be in Binder rather than the component. This PR would introduce a major behavior change that might break other applications.

[ugur-vaadin]

Let's keep the PR in draft until we get a response from the Flow team.