Spark: Fix map value decoding in test helpers for older versions

Problem

The map comparison helpers decode both keys and values with the map key type:

Object actualKey = actualKeys.get(i, convert(keyType));
Object actualValue = actualValues.get(i, convert(keyType));

The value must be decoded with convert(valueType). When the key and value types differ, the current assertions may decode the value incorrectly and do not validate what they appear to validate.

PR #14984 fixes GenericsHelpers for Spark 4.2 and will also fix the duplicate in Spark 4.2 TestHelpers. The same issue remains in both helpers for:

• spark/v3.5
• spark/v4.0
• spark/v4.1

Proposed fix

Update the remaining six occurrences to use:

Object actualValue = actualValues.get(i, convert(valueType));

Add or confirm coverage using a map whose key and value have different Spark types.

Context

Identified while reviewing Spark 4.2 support in #14984: https://github.com/apache/iceberg/pull/14984#discussion_r3788259735