Copilot instructions + test filtering fix

[gregg-miskelly]

This PR:

• Adds copilot-instructions.md with links to documentation
• Adds a READMD.md to the various DebugEngineHost implementations to make that more clear
• Fixes a bug with how DependsOnTest works which made it so that any test with that attribute couldn't be run with a test filter

[Copilot]

Pull request overview

This PR adds Copilot/onboarding documentation (including host-layer guidance and CLI build/test recipes) and updates the xUnit dependency-based test orderer so filtered test runs no longer silently result in “no tests matched”.

Changes:

• Fix DependsOnTest ordering so missing dependencies (e.g., due to dotnet test --filter) don’t cause dependent tests to be removed; instead a warning is logged.
• Add xUnit diagnostic logging support to the test orderer (via IMessageSink when available).
• Add Copilot instructions + several repo docs/READMEs, and include them as Solution Items.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
test/DebuggerTesting/Ordering/DependencyTestOrderer.cs	Adds diagnostic sink support and routes orderer diagnostics into xUnit output.
test/DebuggerTesting/Ordering/DependencyOrderer.cs	Changes dependency handling to ignore missing dependencies (filter-friendly) and adds overridable diagnostic logging hook.
src/MIDebugEngine.sln	Adds .github/docs Solution Items and updates solution header metadata.
src/DebugEngineHost/README.md	Documents the VS-host implementation role and contribution rules.
src/DebugEngineHost.VSCode/README.md	Documents the VS Code/OpenDebugAD7 host implementation role and contribution rules.
src/DebugEngineHost.Stub/README.md	Documents the contract/reference-assembly nature of the Stub project.
docs/Architecture-for-AI.md	Adds an AI-focused architecture/layering overview.
docs/Building-outside-of-VS-for-AI.md	Adds CLI build instructions for Windows and Unix/macOS paths.
docs/CodingStandards-CSharp-for-AI.md	Adds a summary of key C# conventions (anchored on .editorconfig).
docs/DebugEngineHost-for-AI.md	Adds detailed host-contract/implementation rules and map of host types.
docs/RunningUnitTests-outside-of-VS-for-AI.md	Adds CLI instructions for running managed unit tests.
docs/RunningCppTests-outside-of-VS-for-AI.md	Adds CLI instructions for running/diagnosing CppTests and explains the dependency/filtering quirk.
.github/copilot-instructions.md	Adds Copilot instruction entrypoint linking to the new docs.

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