Fix protobuf null inference for Ballerina-generated Struct values

[Delaksan-Sritharan]

Description

Summary

Fixes protobuf null handling for Ballerina-generated google.protobuf.Struct values in the inference engine.

Changes

• Treat structpb.Value objects with unset Kind as JSON null
• Update schema generation to support scalar nulls represented with nil kind
• Update storage inference to classify single-field structs with nil kind as scalar null
• Add regression tests for unset-kind null handling in:
  • pkg/schema
  • pkg/storageinference

Verification

• go test ./pkg/schema ./pkg/storageinference

Issue

Closes: #137

[gemini-code-assist]

Caution

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