@angular/material 19: Datepicker integrated with Timepicker should not reset the time

### Which @angular/* package(s) are the source of the bug?

@angular/material

### Is this a regression?

Yes

### Description

Can be reproduced in the v19 example: https://material.angular.io/components/timepicker/examples#timepicker-datepicker-integration

Steps:
1) use datepicker to select date
2) use timepicker to select time
3) change date - either via picker or type

Expected Result:
Time portion of the value should not change

Actual Result:
Time portion resets to 00:00

### Please provide a link to a minimal reproduction of the bug

https://material.angular.io/components/timepicker/examples#timepicker-datepicker-integration

### Please provide the exception or error you saw

```true

```

### Please provide the environment you discovered this bug in (run `ng version`)

```true
"@angular/material": "19.0.3",
```

### Anything else?

_No response_

Inizia con l’esempio di integrazione di v19 Datepicker e Timepicker collegato nell’issue e riproduci il reset dopo aver modificato la data. Traccia quindi la gestione del valore per questa integrazione e verifica che la modifica della data mantenga l’ora selezionata in precedenza invece di ripristinare 00:00.

Scritto dal modello di indicizzazione a partire dal testo della issue.