google-cloud-go/blob/main/bigquery/storage/managedwriter/adapt: adapt package not working for BigQuery DATETIME type

## Client

BigQuery managedwriter (cloud.google.com/go/bigquery/storage/managedwriter/adapt)

## Environment

Alpine Docker on GKE
go version go1.23.0 linux/amd64

## Code and Dependencies

```go
package main

import (
"time"
"strconv"

bq "cloud.google.com/go/bigquery/storage/managedwriter"
)

func main() {
req := map[string]interface{}{
"entry_time": strconv.FormatInt(time.Now().UTC().UnixMicro(), 10),
}

var result *managedwriter.AppendResult
// sends using adapt-generated schema with DATETIME field
}
```

go.mod

```text
module example

go 1.23.0

require (
cloud.google.com/go/bigquery v1.61.0
)
```

## Expected behavior
It should write to BigQuery table

## Actual behavior

If i use time.Now().Format("2006-01-02 15:04:05.000000"), then i get error at "err = protojson.Unmarshal(eventInBytes, msg)" as it needs int with error "proto: (line 1:15): invalid value for int64 field entryTime: \"2025-07-24 15:27:29.663095\"", but if i use strconv.FormatInt(time.Now().UnixMicro(), 10), then i get error at "res, err := result.FullResponse(ctx)" with error as "Cannot return an invalid datetime value of 1753350970678947 microseconds expressed as civil time, according to civil_time encoding. The range of valid datetime values is [0001-01-01 00:00:00, 9999-12-31 23:59:59.999999] on field entry_time."

In BigQuery field type is DATETIME, and in my code corresponding variable's field type is String

Is there any other way i'm supposed to send it?