[cDAC] Implement GetMetadata for cDAC

[barosiak]

Summary

Implements DacDbiImpl.GetMetadata for the cDAC.

Changes

• DacDbiImpl.cs - Implement GetMetadata.
• CorDbHResults.cs - Add the CORDBG_E_MISSING_METADATA constant.
• EcmaMetadata_1.cs, IEcmaMetadata.cs - Expose GetReadWriteSavedMetadataAddress on the contract.
• EcmaMetadata.md - Document GetReadWriteSavedMetadataAddress.
• DacDbiImplTests.cs - Add tests for the non-dynamic, dynamic, empty-metadata, and null-buffer cases.

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[Copilot]

Pull request overview

Implements DacDbiImpl.GetMetadata in the cDAC legacy Dbi layer by retrieving a target metadata span via the IEcmaMetadata contract (read-only metadata for normal modules; saved read-write metadata for reflection-emit/dynamic modules). The PR also extends the EcmaMetadata contract surface to expose the saved dynamic-metadata address, updates the contract design doc accordingly, and adds unit tests covering key GetMetadata scenarios.

Changes:

• Implement DacDbiImpl.GetMetadata using Loader + EcmaMetadata contracts, including a new HRESULT for missing metadata.
• Expose IEcmaMetadata.GetReadWriteSavedMetadataAddress (implementation + abstractions) and document it.
• Add unit tests for non-dynamic, dynamic, empty-metadata, and null-buffer paths.

Show a summary per file

File	Description
src/native/managed/cdac/tests/UnitTests/DacDbiImplTests.cs	Adds unit tests for DacDbiImpl.GetMetadata covering dynamic/non-dynamic, missing metadata, and null buffer.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/Dbi/DacDbiImpl.cs	Implements GetMetadata by selecting the correct metadata span based on module flags and mapping failures to HRESULTs.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/EcmaMetadata_1.cs	Exposes the saved read-write metadata span accessor needed for reflection-emit modules.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/CorDbHResults.cs	Adds CORDBG_E_MISSING_METADATA HRESULT constant used by GetMetadata.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Contracts/IEcmaMetadata.cs	Extends the EcmaMetadata contract interface with GetReadWriteSavedMetadataAddress.
docs/design/datacontracts/EcmaMetadata.md	Documents the new EcmaMetadata contract API surface.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/Dbi/DacDbiImpl.cs:359

• GetMetadata dereferences pTargetBuffer before validating it (and also accepts vmModule==0), unlike neighboring methods such as GetSymbolsBuffer/GetModuleData which validate pointer arguments up front. Relying on a null dereference to produce an HRESULT is brittle and can become an uncatchable AccessViolation depending on codegen / runtime settings; it also makes the contract less clear for callers. Add explicit argument validation and only write through pTargetBuffer after the null check.

            *pTargetBuffer = default;
            Contracts.ILoader loader = _target.Contracts.Loader;
            Contracts.ModuleHandle handle = loader.GetModuleHandleFromModulePtr(new TargetPointer(vmModule));
            Contracts.ModuleFlags flags = loader.GetFlags(handle);
            Contracts.IEcmaMetadata ecmaMetadata = _target.Contracts.EcmaMetadata;

• Files reviewed: 6/6 changed files
• Comments generated: 0 new