Fix Flaky Test ConverterAwareMappingSpannerEntityReaderTests.readUnmatachableTypesTest()

[pem70]

Summary

This PR fixes a flaky test readUnmatachableTypesTest in
spring-cloud-gcp-data-spanner/src/test/java/com/google/cloud/spring/data/spanner/core/convert/ConverterAwareMappingSpannerEntityReaderTests.java.

The original test asserted an exact exception message:

.hasMessage("Unable to read column from Cloud Spanner results: id");

However, the order in which entity fields are processed is non-deterministic (due to reflection order).
As a result, the first failing field—and thus the message suffix—can vary between runs, leading to intermittent test failures.

Root Cause

• SpannerEntityReader uses reflection over entity fields, which does not guarantee order.
• The test relied on a fixed exception message, making it brittle to iteration changes.

Fix

The test has been updated to:

• Provide explicit read parameters for deterministic field mapping.
• Assert message content rather than full equality.
• Retain a stable suffix check for : id.

Additional

There is a typo from the original test name, which is not modified due to consistency.

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[pem70]

This PR addressed issue #4186