[cDAC] Fix context alignment and content on debug validation paths

[rcj1]

Ensure that buffers passed into debug validation are 16-aligned. Also ensure that native memory is cleared before writing into it.

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[Copilot]

Pull request overview

This PR adjusts cDAC interop/debug-validation code paths to (1) ensure thread/context buffers are 16-byte aligned before being passed to callbacks/legacy implementations and (2) clear native buffers before they’re written/compared, reducing the chance of alignment faults and spurious diffs from uninitialized bytes.

Changes:

• Zero-initialize the temporary aligned buffer used when calling the readThreadContext delegate in mscordaccore_universal.
• Replace byte[] + fixed with NativeMemory.AlignedAlloc(..., 16) in legacy stack-walk validation paths, and clear the native buffer before calling legacy APIs.

Show a summary per file

File	Description
src/native/managed/cdac/mscordaccore_universal/Entrypoints.cs	Clears the temporary aligned buffer before invoking readThreadContext.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/Dbi/DacDbiImpl.cs	Uses 16-byte-aligned native buffers (and clears them) when calling legacy stack-walk APIs for validation.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 5

[rcj1]

/ba-g known sos and x86 issues