[core] Support nested-key-null-strategy in FieldNestedUpdateAgg operator

[PyRSA]

Purpose

[Feature] Add fields.{fieldName}.nested-key-null-strategy configuration for nested_update function
[Feature] Support nested-key null handling strategies in FieldNestedUpdateAgg operator to control behavior when nested-key does not satisfy primary key semantics

Expected behavior when nested-key contains null values or does not satisfy primary key semantics:

In the following examples, nested-key is defined as k0,k1.

case	Input A	Input B	Current Behavior	Expected Behavior
Default behavior (no config)	[(0, null, "A", 1)]	[(0, 1, "B", 2)]	Merge	Merge
MERGE strategy	[(0, null, "A", 1)]	[(0, 1, "B", 2)]	Merge	Merge
IGNORE strategy	[(0, null, "A", 1)]	[(0, 1, "B", 2)]	Merge ❌	Ignore invalid nested-key row
ERROR strategy	[(0, null, "A", 1)]	[(0, 1, "B", 2)]	Merge ❌	Throw exception

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Behavior Definition

• merge
  Merge rows even if nested-key does not satisfy primary key semantics.
  This is equivalent to not configuring nested-key-null-strategy.

• ignore
  Ignore rows whose nested-key does not satisfy primary key semantics.

• error
  Throw an exception when nested-key does not satisfy primary key semantics.

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Tests

• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWithNestedKeyNullStrategyArgumentCheck
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWithoutNestedKeyNullStrategy
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWhenNestedKeyNullUseMergeStrategy
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWhenNestedKeyNullUseIgnoreStrategy
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWhenNestedKeyNullUseThrowErrorStrategy
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWithCountLimitWhenNestedKeyNullUseMergeStrategy
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWithCountLimitWhenNestedKeyNullUseIgnoreStrategy
• org.apache.paimon.mergetree.compact.aggregate.FieldAggregatorTest#testFieldNestedUpdateAggWithCountLimitWhenNestedKeyNullUseThrowErrorStrategy

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API and Format

No Changes.

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Documentation

docs/docs/primary-key-table/merge-engine/aggregation.mdx