Spark: MERGE INTO Statements with only WHEN NOT MATCHED Clauses are always executed at Snapshot Isolation

### Apache Iceberg version

1.8.1 (latest release)

### Query engine

Spark

### Please describe the bug 🐞

When running two concurrent `MERGE INTO` operations on an Apache Iceberg table, I expect them to be **idempotent** -- meaning Iceberg should either detect conflicts and resolve them or fail one of the jobs to prevent data inconsistencies.

However, Iceberg determines the **operation type dynamically** based on the result of the join condition, which can lead to unexpected behavior:
- If a match is found, Iceberg treats it as an **overwrite** operation and fails the second job due to conflicting commits.
- If no match is found, Iceberg considers it an **append** operation and attempts to resolve conflicts by creating a new manifest for appended data, as explained in the [Cost of Retries doc](https://iceberg.apache.org/docs/latest/reliability/#cost-of-retries).

This behavior introduces a problem:
If the dataset is large enough and neither job finds a match, both will proceed with appending data independently, causing **duplicate records**.

#### **Reproduction Steps**
Running the following query in concurrent jobs can result in duplicate data if no matching records exist in `dest`:
```SQL
MERGE INTO dest
USING src
ON dest.id = src.id
WHEN NOT MATCHED THEN
INSERT *
-- even with update action, we'll have the same issue
-- WHEN MATCHED THEN
-- UPDATE SET *
```
I initially expected the **operation type** to be determined by the query itself (i.e., always "append" in the query without `UPDATE` action). However, through testing, I found that Iceberg decides the operation type **at runtime**, based on the actual join results. This makes `MERGE INTO` **non-idempotent**, leading to unintended duplicate inserts.

#### **Expected Behavior**
Iceberg should ensure idempotency for `MERGE INTO`, preventing duplicate data when no matches are found.

#### **Additional Context**
- Iceberg version: 1.8.1
- Iceberg catalog: Glue catalog (type `glue`) with S3 FileIO
- Spark version: 3.5.5

Would love to hear if others have encountered this or if there's a recommended workaround.

### Willingness to contribute

- [ ] I can contribute a fix for this bug independently
- [x] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

Inizia con la riproduzione di Spark MERGE INTO concorrenti su una tabella Iceberg descritta nell’issue, usando la query che contiene solo WHEN NOT MATCHED e le versioni indicate di Spark e Iceberg. Leggi la documentazione Cost of Retries e segui come il risultato della join a runtime seleziona il comportamento di append o overwrite; il lavoro è completato quando i merge concorrenti senza corrispondenze non creano record duplicati e il comportamento è coperto da test di regressione.

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