[Bug]: MySQL GTID mode results in null _metadata_log_file and _metadata_log_position in BigQuery, making record ordering impossible

### Related Template(s)

Cloud Datastream to BigQuery (`DataStream_to_BigQuery` / job label: `cloud_datastream_to_bigquery`)

### Template Version

2025-11-11-00_rc00

### What happened?

#### Expected behavior

When using MySQL Datastream in GTID mode, `_metadata_log_file` and `_metadata_log_position` should be populated in BigQuery — the same as in binlog mode — so that the order of records can be determined for the MERGE operation.

#### Actual behavior

When MySQL Datastream operates in **GTID mode**, the Avro `source_metadata` schema differs from binlog mode. The template hardcodes `"log_file"` and `"log_position"` as the field names to look up ([L150-151](https://github.com/GoogleCloudPlatform/DataflowTemplates/blob/52f460a772452fc5fec1c5c0b9a31ac4eb0c471e/v2/datastream-common/src/main/java/com/google/cloud/teleport/v2/datastream/transforms/FormatDatastreamRecordToJson.java#L150-L151)), but in GTID mode Avro, these fields are named `"binlog_file"` and `"binlog_position"`. Because `getSourceMetadata()` returns `null` when the field is absent from the schema ([L243-L251](https://github.com/GoogleCloudPlatform/DataflowTemplates/blob/52f460a772452fc5fec1c5c0b9a31ac4eb0c471e/v2/datastream-common/src/main/java/com/google/cloud/teleport/v2/datastream/transforms/FormatDatastreamRecordToJson.java#L243-L251)), both `_metadata_log_file` and `_metadata_log_position` are written as **null** in BigQuery.

Binlog mode Avro:

```json
{
"read_method": "mysql-cdc-binlog",
"source_metadata": {
"log_file": "mysql-bin-changelog.072901",
"log_position": 33822
}
}
```

GTID mode Avro:

```json
{
"read_method": "cdc",
"source_metadata": {
"gtid": "92e3e612-66e2-11f0-a5a7-0e004f023b83:22073",
"binlog_file": "mysql-bin-changelog.073728",
"binlog_position": 147862
}
}
```

In other words, the ordering metadata is present in GTID-mode Avro, but the template does not extract it because it only supports the binlog-mode field names.

#### Impact

- `_metadata_log_file` and `_metadata_log_position` are always null for records ingested via GTID mode
- When multiple CDC changes for the same key occur within the same timestamp boundary, the template cannot use binlog ordering to deterministically rank them, which can break deduplication and upsert correctness in the generated `MERGE` logic.

#### Suggested fix

When `source_metadata.log_file` / `source_metadata.log_position` are absent, the template should also check for:

- `source_metadata.binlog_file`
- `source_metadata.binlog_position`

and map them to:

- `_metadata_log_file`
- `_metadata_log_position`

This would preserve ordering semantics for MySQL GTID-mode Datastream sources.

> [!NOTE]
> Note: The related issue where GTID mode triggers the Oracle default sort branch (_metadata_scn) can be mitigated by setting the `datastreamSourceType=mysql` override parameter.
> However, even with that workaround applied, _metadata_log_file and _metadata_log_position remain null. This report focuses on that remaining problem.

### Relevant log output

```shell

```