Data: TestWriterMetrics.testMaxColumnsWithDefaultOverride does not verify bounds

**Apache Iceberg version**

main @ 5f3d3c560

**Query engine**

None — engine-agnostic. Defined in `data/`, run by the Flink and Spark implementations of `TestWriterMetrics`.

**Please describe the bug**

`TestWriterMetrics.testMaxColumnsWithDefaultOverride()` (`data/src/test/java/org/apache/iceberg/io/TestWriterMetrics.java` line 344-352) does not verify what its name and comment describe: that setting `write.metadata.metrics.default` explicitly bypasses the max-inferred-column limit, so all 101 columns keep metrics.

- Line 346 reads lower bounds from `dataFile.upperBounds()`. `dataFile.lowerBounds()` is never called, so lower bounds go unchecked.
- The loop variable at line 347 is unused; both assertions hardcode `get(1)`. Only column 1 is checked, 101 times.

The test stays green even if the other 100 columns lose their metrics — the exact regression it exists to catch.

**Steps to reproduce**

Suppress the `configuredDefault != null` bypass in `MetricsConfig.from()` (`core/src/main/java/org/apache/iceberg/MetricsConfig.java` line 265) so `limitFieldIds` truncates to 100 columns.

Expected: the test fails. Actual: it passes on both ORC and Parquet.

**Additional context**

Sibling tests in the same file show the intended form: `testMaxColumnsBounded` (line 307) asserts the bounds map size, and `testMaxColumns` (line 241-269) uses its loop variable to check `lowerBounds()` and `upperBounds()` separately.

Introduced in #3959 (2022-02); the assertions have had this shape since.

Start in data/src/test/java/org/apache/iceberg/io/TestWriterMetrics.java at testMaxColumnsWithDefaultOverride, then compare testMaxColumns and testMaxColumnsBounded. Run the TestWriterMetrics tests for the Flink and Spark implementations; done means all 101 columns have both lower- and upper-bound metrics, and the test detects truncation when the MetricsConfig bypass is suppressed.

Written by the indexing model from the issue text.