Add zero and copy dispatches for PreallocationTools types

[ChrisRackauckas-Claude]

Summary

This PR adds Base.zero and Base.copy dispatches for all PreallocationTools cache types, enabling these standard Julia operations on cache structures.

Changes

• Added Base.zero methods for DiffCache, FixedSizeDiffCache, LazyBufferCache, and GeneralLazyBufferCache
• Added Base.copy methods for all four cache types
• Modified struct definitions to allow dual_du to be Nothing (for compatibility when ForwardDiff extension isn't loaded)
• Added comprehensive test suite in test/test_zero_copy.jl

Implementation Details

zero methods

• For DiffCache and FixedSizeDiffCache: Creates new caches with zeroed arrays
• For LazyBufferCache and GeneralLazyBufferCache: Creates new empty caches with the same configuration

copy methods

• Creates deep copies of all internal arrays and buffers
• Ensures copied caches are independent (modifications don't affect originals)
• Properly handles cases where dual_du might be nothing

Testing

• Added comprehensive tests for all four cache types
• Tests verify that zero creates properly zeroed structures
• Tests verify that copy creates independent copies (not references)
• Tests handle both cases where ForwardDiff extension is/isn't loaded
• All new tests pass successfully

Example Usage

using PreallocationTools

# Create a cache
cache = DiffCache(rand(5), 3)

# Create a zeroed version
zero_cache = zero(cache)  # All arrays are zeroed

# Create an independent copy
copy_cache = copy(cache)
copy_cache.du[1] = -999  # Doesn't affect original cache

Compatibility

• Maintains full backward compatibility
• Works correctly with and without ForwardDiff extension loaded
• All existing tests still pass

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