Make UnmanagedMemoryHandle members readonly and improve pool finalization tests

[JimBobSquarePants]

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Description

Fixes #1887
Fixes #2342

Marked UnmanagedMemoryHandle properties and methods as readonly for improved immutability and performance. Refactored UniformUnmanagedPoolMemoryAllocatorTests to remove platform/CI checks, use culture-invariant parsing, and enhance finalization tests to assert handle pooling via handle count invariants rather than memory content or pointer values.

[Copilot]

Pull request overview

This PR improves the UnmanagedMemoryHandle struct and refactors finalization tests to be more robust and reliable across platforms. The changes address issues #1887 and #2342 related to flaky finalization tests on macOS and ARM64.

Changes:

• Added readonly modifiers to UnmanagedMemoryHandle properties and methods for improved struct immutability
• Removed platform-specific and CI-only test skips that were masking finalization test issues
• Refactored finalization tests to use handle count invariants instead of unreliable sentinel value checks

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
src/ImageSharp/Memory/Allocators/Internals/UnmanagedMemoryHandle.cs	Added readonly modifiers to properties (Handle, IsInvalid, IsValid, Pointer) and methods (Equals, GetHashCode) for better struct semantics
tests/ImageSharp.Tests/Memory/Allocators/UniformUnmanagedPoolMemoryAllocatorTests.cs	Removed platform/CI checks, added culture-invariant parsing, and refactored finalization tests to verify pooling via TotalOutstandingHandles count instead of memory content comparison

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