[cDAC] Add Hijack DacDbi API

[rcj1]

• Adds a StackPusher utility to push arbitrary bytes onto the stack with or without alignment
• Adds an IntegerArgPlacer utility to place integer args according to the native ABI
• Adds TargetPointer PrepareExceptionHijack(byte[] context, TargetPointer vmThread, byte[]? exceptionRecord, int reason, TargetPointer userData); to the Debugger contract
• Adds context-write delegate to cDAC
• Implements Hijack DacDbi API

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Pull request overview

This PR extends the cDAC stack-walking/debugger plumbing to support thread-context writes (SetThreadContext), exposes hijack function address via the Debugger contract, surfaces OS exception record/context pointers through the Thread contract, and implements the managed DacDbi Hijack path using new stack/ABI helper utilities.

Changes:

• Add a thread-context write callback/delegate across the cDAC initialization surface (native header, native host glue, managed entrypoints, and Target/ContractDescriptorTarget).
• Extend Thread/Debugger contracts and data descriptors to expose OS exception record/context pointers and to return the hijack function address.
• Implement managed DacDbi Hijack, plus new helper utilities (StackPusher, IntegerArgPlacer) and add/adjust unit tests + design docs.

Show a summary per file

File	Description
src/tools/StressLogAnalyzer/src/Program.cs	Plumbs new SetThreadContext delegate slot (not implemented for this tool).
src/native/managed/cdac/tests/UnitTests/ThreadTests.cs	Adds coverage for OS exception record/context exposure via ThreadData.
src/native/managed/cdac/tests/UnitTests/PlatformContextTests.cs	Adds coverage for UnsetSingleStepFlag behavior across architectures.
src/native/managed/cdac/tests/UnitTests/MockDescriptors/MockDescriptors.Thread.cs	Extends mock ExceptionInfo layout with OS record/context pointer fields.
src/native/managed/cdac/tests/UnitTests/DebuggerTests.cs	Adds tests for GetHijackAddress() behavior.
src/native/managed/cdac/tests/UnitTests/ClrDataExceptionStateTests.cs	Updates ThreadData construction for new fields.
src/native/managed/cdac/tests/TestInfrastructure/TestPlaceholderTarget.cs	Adds stub override for new TrySetThreadContext.
src/native/managed/cdac/tests/TestInfrastructure/DumpTestBase.cs	Wires SetThreadContext delegate for dump test target creation.
src/native/managed/cdac/tests/TestInfrastructure/ContractDescriptor/ContractDescriptorBuilder.cs	Updates ContractDescriptorTarget.TryCreate call with SetThreadContext delegate.
src/native/managed/cdac/tests/DataGenerator/TestTarget.cs	Adds stub override for new TrySetThreadContext.
src/native/managed/cdac/scripts/DumpHelpers.cs	Updates script helper target creation signature for SetThreadContext.
src/native/managed/cdac/mscordaccore_universal/Entrypoints.cs	Adds unmanaged callback for writing thread context and forwards into ContractDescriptorTarget.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader/ContractDescriptorTarget.cs	Adds SetThreadContext delegate support and implements TrySetThreadContext.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/Dbi/DacDbiImpl.cs	Implements managed Hijack and adds helpers for EXCEPTION_RECORD sizing/writes.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/ExceptionInfo.cs	Adds fields for OS EXCEPTION_RECORD/CONTEXT pointers.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Thread_1.cs	Populates new ThreadData fields from ExceptionInfo.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Debugger_1.cs	Implements IDebugger.GetHijackAddress().
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/IPlatformContext.cs	Adds UnsetSingleStepFlag() API to platform contexts.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/IPlatformAgnosticContext.cs	Adds UnsetSingleStepFlag() API + dispatch.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/ContextHolder.cs	Forwards UnsetSingleStepFlag() to the held context.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/X86Context.cs	Implements UnsetSingleStepFlag() for x86.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/AMD64Context.cs	Implements UnsetSingleStepFlag() for x64.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/ARM64Context.cs	Implements UnsetSingleStepFlag() for arm64.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/ARMContext.cs	Throws for UnsetSingleStepFlag() (emulated single-step).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/RISCV64Context.cs	Throws for UnsetSingleStepFlag() (emulated single-step).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/Context/LoongArch64Context.cs	Throws for UnsetSingleStepFlag() (emulated single-step).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackPusher.cs	New helper for stack writes with optional ABI alignment.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/IntegerArgPlacer.cs	New helper for placing integer arguments per ABI (regs/stack).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Target.cs	Adds TrySetThreadContext API to the target abstraction.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Contracts/IThread.cs	Extends ThreadData with exception-in-progress + OS record pointers.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Contracts/IDebugger.cs	Adds GetHijackAddress() API to debugger contract.
src/native/managed/cdac/inc/cdac_reader.h	Extends native init signature with optional write_thread_context callback.
src/coreclr/vm/exinfo.h	Adds cDAC offsets for ExInfo’s OS exception record/context pointers.
src/coreclr/vm/datadescriptor/datadescriptor.inc	Adds ExceptionInfo fields for OS exception record/context pointers.
src/coreclr/vm/cdacstress.cpp	Updates in-proc stress init call signature for new callback parameter.
src/coreclr/debug/daccess/cdac.cpp	Wires ICorDebugMutableDataTarget::SetThreadContext into cDAC init.
docs/design/datacontracts/Thread.md	Documents new ThreadData fields and descriptor dependencies.
docs/design/datacontracts/Debugger.md	Documents new GetHijackAddress() API.

Copilot's findings

• Files reviewed: 38/38 changed files
• Comments generated: 7

[Copilot]

Copilot's findings

• Files reviewed: 38/38 changed files
• Comments generated: 6

[Copilot]

Copilot's findings

• Files reviewed: 37/37 changed files
• Comments generated: 5

[noahfalk]

This looks good to me modulo a few suggestions inline.

[Copilot]

Copilot's findings

• Files reviewed: 37/37 changed files
• Comments generated: 4

[max-charlamb]

lgtm mod few comments

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