[Improvement]: Add non-partitioned table test coverage for mixed-hive rewrite and overwrite operations

### Search before asking

- [x] I have searched in the [issues](https://github.com/apache/amoro/issues?q=is%3Aissue) and found no similar issues.

### What would you like to be improved?

The mixed-hive test suite already covers several rewrite and overwrite scenarios, but multiple test methods still contain the same TODO: add cases for tables without partition spec.

Examples can be found in:
- `amoro-format-mixed/amoro-mixed-hive/src/test/java/org/apache/amoro/hive/op/TestRewriteFiles.java`
- `amoro-format-mixed/amoro-mixed-hive/src/test/java/org/apache/amoro/hive/op/TestOverwriteFiles.java`

Currently these cases are guarded by `Assume.assumeTrue(isPartitionedTable())`, so behavior for non-partitioned tables is not covered well enough.

### How should we improve?

We should add non-partitioned table test coverage for mixed-hive rewrite and overwrite operations.

A contributor can start by:
- identifying the existing partitioned-only test cases in `TestRewriteFiles` and `TestOverwriteFiles`
- adding equivalent or adapted cases for tables without partition spec
- verifying whether non-partitioned tables should succeed or throw the expected exception in each scenario
- keeping the new tests independent and easy to understand

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Subtasks

- [ ] Review the existing partitioned-table cases in `TestRewriteFiles`
- [ ] Review the existing partitioned-table cases in `TestOverwriteFiles`
- [ ] Add non-partitioned table coverage for the missing scenarios
- [ ] Run the relevant mixed-hive tests to validate the expected behavior

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)