[Bug]: GcsTextToCloudSpanner hangs in infinite loop when invalidOutputPath is omitted

### Related Template(s)

gs://dataflow-templates-europe-west2/latest/GCS_Text_to_Cloud_Spanner

### Template Version

Beam SDK Version: 2.72.0

### What happened?

**Description**
In the `GCS_Text_to_Cloud_Spanner` template, the `invalidOutputPath` parameter in `TextImportPipeline.java` is currently set as optional with an empty default string.

If a user triggers the pipeline with malformed source data (e.g., a decimal value that exceeds Spanner's limits) and does not provide an `invalidOutputPath`, the Dead Letter Queue attempts to write the failed mutation to a null/empty GCS bucket.

Because there is no pre-flight validation to check if the error bucket exists, the worker crashes during the `Finalize` step. Dataflow then indefinitely retries this failed bundle, causing the job to hang permanently at 100% completion rather than failing gracefully, leading to runaway compute costs.

**Steps to reproduce**
1. Launch the `GCS_Text_to_Cloud_Spanner` template.
2. Provide a source CSV that contains a value that Spanner will reject (e.g., a NUMERIC field with 15 decimal places).
3. Omit the `invalidOutputPath` parameter.
4. Observe the worker getting stuck in an infinite retry loop during the `Finalize` step.

**Expected behavior**
The parameter `invalidOutputPath` should be strictly required. If it is omitted, the pipeline should throw a validation exception and fail immediately at initialization before spinning up workers.

**Additional context**
I have already written a fix for this by setting `optional = false` and removing the empty default string on the parameter. I will open a Pull Request linking to this issue shortly!

### Relevant log output

```shell
INFO 2026-04-21T08:52:37.287Z Finalizing 1 file results

INFO 2026-04-21T08:52:37.311Z Finalizing for destination null num shards 1.

INFO 2026-04-21T08:52:37.334Z Will copy temporary file FileResult{tempFilename=/.temp-beam-2b3a3eea-9e34-4e47-a2a2-43fa6afba2ed/9226b88e6be1a28c-336b-447b-b512-a4ee1140d53b, shard=0, window=GlobalWindow, paneInfo=PaneInfo.NO_FIRING} to final location /-00000-of-00001-7ab1bac4-7dd4-46ea-b816-30bf2fd991eb-spanner-depth-0.csv

{

"insertId": "7194243126373549299:152328:0:59730",

"jsonPayload": {

"work": "7202850336977575572",

"job": "2026-04-21_01_46_59-15019665261852597760",

"message": "Will copy temporary file FileResult{tempFilename=/.temp-beam-2b3a3eea-9e34-4e47-a2a2-43fa6afba2ed/9226b88e6be1a28c-336b-447b-b512-a4ee1140d53b, shard=0, window=GlobalWindow, paneInfo=PaneInfo.NO_FIRING} to final location /-00000-of-00001-7ab1bac4-7dd4-46ea-b816-30bf2fd991eb-spanner-depth-0.csv",

"worker": "option-hedge-2026-04-21-1-04210146-f7qs-harness-7csj",

"thread": "23",

"step": "TextImportTransform/Write failed Spanner records 0/WriteFiles/FinalizeTempFileBundles/Finalize/ParMultiDo(Finalize)",

"logger": "org.apache.beam.sdk.io.FileBasedSink",

"instruction": "process_bundle-7202850336977575572-96",

"portability_worker_id": "sdk-0-0"

},

"resource": {

"type": "dataflow_step",

"labels": {

"job_name": "option_hedge_2026-04-21_1776760795-processing-job",

"step_id": "TextImportTransform/Write failed Spanner records 0/WriteFiles/FinalizeTempFileBundles/Finalize/ParMultiDo(Finalize)",

"job_id": "2026-04-21_01_46_59-15019665261852597760",

"region": "europe-west2",

"project_id": "xo-attara-portal"

}

},

"timestamp": "2026-04-21T08:52:37.334Z",

"severity": "INFO",

"labels": {

"compute.googleapis.com/resource_name": "option-hedge-2026-04-21-1-04210146-f7qs-harness-7csj",

"dataflow.googleapis.com/job_name": "option_hedge_2026-04-21_1776760795-processing-job",

"dataflow.googleapis.com/job_id": "2026-04-21_01_46_59-15019665261852597760",

"dataflow.googleapis.com/region": "europe-west2",

"compute.googleapis.com/resource_type": "instance",

"compute.googleapis.com/resource_id": "7194243126373549299",

"dataflow.googleapis.com/log_type": "supportability"

},

"logName": "projects/xo-attara-portal/logs/dataflow.googleapis.com%2Fworker",

"receiveTimestamp": "2026-04-21T08:52:57.067702626Z"

}
```