What is the bug?
In ExplainSMPolicy.kt, the toXContent() method incorrectly uses .field() for the nullable creation field instead of .optionalField(). This is inconsistent with how the deletion field is handled on the same line and with the reference implementation in SMMetadata.toXContent(). Using .field() with a nullable value can cause incorrect serialization behavior when the creation workflow metadata is null.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Create a snapshot management policy where the creation workflow metadata can be null
2. Call the Explain API to retrieve the policy metadata
3. Observe the JSON response when metadata.creation is null
4. See incorrect serialization behavior due to .field() being used instead of .optionalField()

What is the expected behavior?
The creation field should use .optionalField() to properly handle null values, just like the deletion field does. When creation is null, it should be omitted from the JSON response or handled gracefully rather than causing potential serialization issues.

Expected code (line 39 in ExplainSMPolicy.kt):

.optionalField(SMMetadata.CREATION_FIELD, it.creation)

Current (incorrect) code:

.field(SMMetadata.CREATION_FIELD, it.creation)

What is your host/environment?

• OS: N/A (Code-level bug in serialization)
• Version: 3.3.0-SNAPSHOT
• Plugins: OpenSearch Index Management Plugin - Snapshot Management component

Do you have any screenshots?
N/A - This is a code-level serialization bug

Do you have any additional context?

• Location: src/main/kotlin/org/opensearch/indexmanagement/snapshotmanagement/model/ExplainSMPolicy.kt, line 39
• Inconsistency: The deletion field on line 40 correctly uses .optionalField()
• Reference: SMMetadata.kt line 55 shows the correct implementation using .optionalField() for the creation field
• Field Type: creation is declared as nullable WorkflowMetadata? in SMMetadata (line 42 of SMMetadata.kt)
• Root Cause: The creation field was recently changed to be optional (nullable) as part of version 3.3.0 changes, but the ExplainSMPolicy serialization was not updated accordingly