[cDAC] Implement DacDbi GetVarArgSig API

[Copilot]

Adding Signature cDAC APIs

    TargetPointer GetVarArgArgsBase(TargetPointer vaSigCookieAddr) => throw new NotImplementedException();
    void GetVarArgSignature(TargetPointer vaSigCookieAddr, out TargetPointer signatureAddress, out uint signatureLength) => throw new NotImplementedException();

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[Copilot]

Pull request overview

This PR extends the cDAC Signature contract and CoreCLR data descriptors to support DAC/DBI vararg cookie decoding (arg base + raw signature pointer/length), and wires that through the managed legacy DACDBI implementation, with accompanying documentation and unit tests.

Changes:

• Extend ISignature and implement vararg cookie helpers in Signature_1 (GetVarArgArgsBase, GetVarArgSignature).
• Introduce a new VASigCookie data descriptor (native) and a managed reader type to expose sizeOfArgs and the embedded Signature pointer/length.
• Update the legacy DacDbiImpl.GetVarArgSig to use the new contract APIs and add docs/tests.

Show a summary per file

File	Description
src/native/managed/cdac/tests/SignatureTests.cs	Adds unit tests for vararg cookie signature/arg-base APIs and error paths.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/Dbi/DacDbiImpl.cs	Implements GetVarArgSig via cDAC Signature contract (with DEBUG cross-validation).
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Data/VASigCookie.cs	Adds managed data reader for the VASigCookie descriptor fields.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Signature/Signature_1.cs	Implements the new vararg cookie APIs on the v1 Signature contract.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/DataType.cs	Adds DataType.VASigCookie.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Contracts/ISignature.cs	Adds new public members to ISignature.
src/coreclr/vm/siginfo.hpp	Grants cDAC data access to Signature’s private fields for cookie decoding.
src/coreclr/vm/datadescriptor/datadescriptor.inc	Adds the VASigCookie native data descriptor definition.
src/coreclr/vm/ceeload.h	Adds cdac_data<VASigCookie> offsets to reach Signature’s pointer/length.
docs/design/datacontracts/Signature.md	Documents new APIs, new data descriptor, and vararg cookie decoding behavior.

Copilot's findings

Comments suppressed due to low confidence (1)

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/DataType.cs:183

• DataType is a public enum with implicit sequential values. Inserting VASigCookie here will shift the numeric values of all subsequent members (the GC Data Types), which is a breaking change for any consumers persisting/casting these values or for any protocol that relies on stable IDs. Prefer appending new members at the end, or assigning an explicit value to the new member (and/or explicitly pinning the existing values) to avoid renumbering.

    ComInterfaceEntry,
    InternalComInterfaceDispatch,
    AuxiliarySymbolInfo,
    VASigCookie,
    CodeRangeMapRangeList,

    /* GC Data Types */

    GCHeap,

• Files reviewed: 10/10 changed files
• Comments generated: 4