Fix crash in nested decoder reuse when codingPath is shorter than expected

[someotherkyle]

Summary

Adds a bounds check in reuseNestedDecoder to prevent a fatal crash (Range requires lowerBound <= upperBound) when a
cached nested decoder's codingPath is shorter than the parent decoder's codingPath.count.

Problem

reset() calls codingPath.replaceSubrange(codingPathIndexToReplace..., with:) without verifying that
codingPathIndexToReplace <= codingPath.count. When the reusable nested decoder's codingPath has been shortened by a prior
decode cycle — specifically through the nil-key path in singleValue's compoundValue(for: nil) which calls removeSubrange
— the index can exceed the array bounds, producing a fatal error.

This occurs in production when decoding deeply nested structures (7+ levels) where:

• Array iteration reuses decoder chains across elements
• Optional.init(from:) goes through singleValueContainer → compoundValue(T) → reuseNestedDecoder(for: nil), which
  truncates the cached decoder's codingPath via removeSubrange
• A subsequent decode at the same or deeper level reuses the shortened decoder with a codingPathIndexToReplace that now
  exceeds its codingPath.count

The crash is intermittent because it depends on the specific nesting depth, the presence/absence of optional compound
fields, and whether the decoder reuse chain was populated by prior elements with different depth characteristics.

Stack trace from production

Fatal error: Range requires lowerBound <= upperBound

frame #3: PartialRangeFrom<τ_0_0>.relative(to:)
frame #4: RangeReplaceableCollection.replaceSubrange(_:with:)
frame #5: MessagePackValueToSwiftValueDecoder.reset(codingPathIndexToReplace=7)
frame #6: MessagePackValueToSwiftValueDecoder.reuseNestedDecoder(codingKey="binloc")
frame #7: MessagePackValueToSwiftValueDecoder.compoundValue<String, String>

Fix

A single guard condition added to reuseNestedDecoder in MessagePackValueToSwiftValueDecoder.swift:

if let reusableNestedDecoder,
reusableNestedDecoder.codingPath.count >= codingPath.count // added
{

When the cached decoder's codingPath is too short to safely call replaceSubrange(codingPath.count..., ...), the decoder
falls through to create a fresh instance instead of crashing. This trades a small amount of reuse for correctness.

Tests

Added NestedDecoderReuseCodingPathTests.swift with 4 tests exercising the decoder reuse optimization across:

• Array elements with optional compound fields and generic Decodable dispatch
• Deeply nested structures (Page → PageValue → Order → OrderData → Item → fields) mirroring real-world usage
• Mixed nil/non-nil optional presence across array elements and pages
• Generic string fields forced through the compoundValue → reuseNestedDecoder path

All 175 tests pass (171 existing + 4 new).