Add unit test for lastKeySearchHint inheritance in nested decoders

[Copilot]

Demonstrates potential optimization inefficiency where lastKeySearchHint is not reset when creating subDecoders, causing inherited hints to point to invalid offsets in child contexts.

Test Coverage

• Simple nested objects: Parent with multiple sequential keys before nested object to establish hint value
• Deep nesting (4 levels): Demonstrates compounding effect where each level inherits progressively invalid hints
• Sequential key access: Triggers the hint mechanism to expose wraparound behavior

Behavior Verified

The test confirms current implementation handles nested objects correctly despite the issue. Wraparound logic in valueOffset(forKey:convertingSnakeCasing:in:hint:) prevents incorrect results but forces unnecessary wraparound searches when hints reference parent index space.

func subDecoder(offsetBy offset: Int) -> _JSONDecoder {
    let subDescription = self.description.unsafeReadOnlySubDescription(offset: offset)
    return _JSONDecoder(description: subDescription, codingPath: _codingPath, pointer: pointer, settings: settings)
    // lastKeySearchHint inherited here - points to parent's index space
}

Child decoders should initialize with lastKeySearchHint = Constants.firstArrayObjectChildOffset rather than inheriting parent's offset.

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