Fix for @version field value ignored when updating data.

[ruthst00]

Fixes #1689

Root Cause

In DomainObjectReader.doMerge() (used for HTTP PATCH / JSON Merge Patch), the version field sent by the client was being silently stripped from the JSON node before Jackson applied it to the target object. This happened because MappedJacksonProperties.isWritableField() returns false for version properties, and doMerge() removes any non-writable field from the JSON node:

if (!mappedProperties.isWritableField(fieldName)) {
    i.remove();  // ← version field was removed here
    continue;
}

As a result, the existing version from the database was always kept, so the underlying store's optimistic locking mechanism never saw a version mismatch — even when the client sent a wrong version number.

Fix

DomainObjectReader.doMerge() — Instead of unconditionally removing all non-writable fields, the fix checks if the non-writable field is the version property. If it is, the field is kept in the JSON node so Jackson can apply it to the target object, enabling the store's optimistic locking to detect mismatches. The id field is still stripped to prevent clients from changing resource identity.

if (!mappedProperties.isWritableField(fieldName)) {
    // Allow the version field to pass through so that the underlying store's optimistic locking
    // mechanism can detect version mismatches (GH-1689). The id field is still stripped to
    // prevent clients from changing the identity of the resource.
    PersistentProperty<?> nonWritable = mappedProperties.getPersistentProperty(fieldName);
    if (nonWritable == null || !nonWritable.isVersionProperty()) {
        i.remove();
    }
    continue;
}

Behavior preserved

• HTTP PUT: retainIdentifierAndVersion() still overwrites the client's version with the server's version before deserialization, so PUT cannot mutate the version (existing tests doesNotWipeIdAndVersionPropertyForPut and doesNotAllowMutatingIdAndVersionViaPutBody still pass).
• JSON Patch: JsonPointerMapping.forWrite() still rejects writes to the version property via JSON Patch operations (existing test forWriteRejectsVersionProperty still passes).
• HTTP PATCH (JSON Merge Patch): The client-provided version is now applied to the object, allowing the store to throw OptimisticLockException on version mismatch.

Tests added

Two new tests in DomainObjectReaderUnitTests:

• appliesVersionFromClientForPatch() — verifies the version from the client is applied during PATCH
• doesNotAllowMutatingVersionViaPutBody() — verifies the version cannot be changed via PUT

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