API: Compute variant sizes without full serialization in VariantTestUtil

[ArnabKarmakar1108]

Feature Request / Improvement

Problem

VariantTestUtil.createArray pre-computes the data section size by summing sizeInBytes() for each element before allocating the output buffer:

// api/src/test/java/org/apache/iceberg/variants/VariantTestUtil.java:273
for (VariantValue value : values) {
  // TODO: produce size for every variant without serializing
  dataSize += value.sizeInBytes();
}

This works for pre-serialized values (SerializedPrimitive, SerializedShortString, etc.) where sizeInBytes() is just buffer().remaining(). It breaks down for nested or non-buffer-backed values — e.g. a SerializedArray passed as an element may not reflect the size needed when embedded in a parent array without already being fully serialized.

The same pattern exists in createObject (line 220) without a TODO, but would benefit from the same fix.

Proposed behavior

Provide a way to compute the serialized byte size of any VariantValue without building the full parent buffer first. Options to consider:

1. Add a sizeInBytes() implementation (or helper) that recursively computes size from type/metadata for all variant physical types
2. Add a VariantTestUtil.sizeInBytes(VariantValue) helper that handles nested arrays/objects correctly
3. Refactor createArray / createObject to use it and remove the TODO

createArray should then allocate the correct buffer size for mixed and nested element types without redundant serialization work.

Scope

• Module: api (test utility — VariantTestUtil.java)
• May touch: VariantValue / serialized variant classes if a shared size API is added
• Tests: TestSerializedArray, TestSerializedObject, and any tests using VariantTestUtil.createArray with nested values
• No production API changes unless maintainers prefer a public sizeInBytes improvement in core variant types

Notes

• Test-only improvement unless core variant sizing is generalized.
• createObject uses the same sizeInBytes() loop — consider fixing both together.
• Related tests already cover nested arrays (TestSerializedArray.testArrayOfMixedTypes).

Query engine

None

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time