Deduplicate base interfaces from another assembly in COM generator

[jtschuster]

Fixes cross-assembly interface inheritance.

Deduplicate external ComInterfaceInfos in the pipeline.

Don't remove external interface methods in the generator pipeline - these are required to create the new VTable struct type introduced in #116289.

Creates a new SourceAvailableIncrementalMethodStubGenerationContext for methods with source available and use IncrementalMethodStubGenerationContext as a base for externally defined methods.

Moves the ComInterfaces from SharedTypes to the Common folder, and defines a limited set of base interfaces in SharedTypes for interfaces in ComInterfaceGenerator to derived from. Adds a number of tests for different levels of inheritance across assemblies.

[Copilot]

Pull Request Overview

This PR fixes cross-assembly interface inheritance in the COM interface generator by properly handling external interface references and enabling deduplication of base interfaces from other assemblies. The key changes include refactoring the generation context hierarchy, moving shared interfaces to a common location, and adding comprehensive tests for cross-assembly inheritance scenarios.

Key changes:

• Adds deduplication of external ComInterfaceInfos to prevent duplicate processing of base interfaces
• Creates new context classes to separate source-available from sourceless method generation
• Moves shared COM interfaces from TestAssets/SharedTypes to Common folder for better organization

Reviewed Changes

Copilot reviewed 25 out of 72 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
SharedTypes.csproj	Removes code generation exclusion to enable proper interface inheritance testing
IExternalBase.cs / IExternalDerived.cs	Adds base COM interfaces for cross-assembly inheritance testing
CrossAssemblyInheritanceTests.cs	Comprehensive test suite validating VTable layout consistency across assemblies
ComInterfaceGenerator.cs	Implements external interface deduplication and refactors stub generation logic
IncrementalMethodStubGenerationContext.cs	Creates inheritance hierarchy for source-available vs. sourceless contexts
ComInterfaceInfo.cs	Adds equality comparer for external interface deduplication