Add .github/copilot-review-instructions.md to reduce noisy automated PR comments

[YunchuWang]

Summary

Adds a .github/copilot-review-instructions.md\ file to guide the GitHub Copilot PR review bot, reducing false-positive and low-signal automated comments.

Motivation

In recent PRs (e.g., #682), the Copilot review bot generated several noisy comments:

• Falsely claiming valid C# 7.2 named-argument syntax was invalid
• Style suggestions (ternary, nested if) on unchanged code outside the PR diff
• Suggesting converting \�irtual\ methods to \�bstract\ (a breaking change)
• Suggesting tests for unmodified code paths
• Comments from \github-code-quality\ bot on pre-existing code

What this file does

Per GitHub docs, .github/copilot-review-instructions.md\ provides custom instructions that the Copilot code review bot reads when reviewing PRs. This is separate from .github/copilot-instructions.md\ (which controls Chat/Completion).

Key rules added

• Scope: Only review lines in the PR diff; skip unchanged code
• Language accuracy: Don't flag valid modern C# syntax (\LangVersion latest\ / C# 14)
• Comment quality: Prioritize correctness bugs over style nitpicks
• Virtual methods: Recognize the deliberate \�irtual\ + default-impl pattern
• Nullability: Verify base declarations before flagging override mismatches
• Test code: Don't suggest tests for unmodified code paths
• Consolidation: Defers to existing \copilot-instructions.md\ review rules to avoid drift

Review quality

This file was reviewed by the code-review-agent and scored 94/100 with 0 blockers, 0 high, 0 medium issues. Only 2 nit-level observations remain (sealed scope consistency in test section, C# version parenthetical staleness).

[Copilot]

Pull request overview

Adds a repository-level Copilot code review instruction file to constrain automated PR review feedback to the diff and reduce recurring false-positive / low-signal comments in this codebase.

Changes:

• Introduces .github/copilot-review-instructions.md with scope, language-accuracy, and comment-quality guidance for Copilot PR reviews.
• Documents repo-specific review heuristics around virtual-method default implementations, nullability checks, and test-related feedback.
• References .github/copilot-instructions.md for consolidated, non-duplicated baseline rules.

[Copilot]

The “sealed” guidance in the Test Code section is narrower than the earlier guidance (which allows commenting when visibility/inheritance changes). To avoid inconsistent bot behavior, consider aligning this bullet with the earlier rule (e.g., allow comments when a test helper class is newly introduced or its inheritance/visibility was changed in the PR).

Suggested change

	- Test helper classes and mock implementations in `test/` directories are internal to the test project. Apply the same `sealed` requirement from `copilot-instructions.md` only to new classes introduced in the PR.
	- Test helper classes and mock implementations in `test/` directories are internal to the test project. Apply the same `sealed` requirement from `copilot-instructions.md` only when such a class is newly introduced in the PR or its visibility/inheritance was changed in the PR.

[Copilot]

The statement tying <LangVersion>latest</LangVersion> specifically to “.NET 10 (currently C# 14)” is likely to go stale and is slightly inaccurate for this repo: LangVersion is set globally (Directory.Build.props), and the exact C# version implied by latest changes over time. Consider rephrasing to reference the repo setting without naming a specific C# version (or link to Directory.Build.props).

Suggested change

	- **Verify C# syntax claims before posting.** This repository uses `<LangVersion>latest</LangVersion>` on .NET 10 (currently C# 14). Do not flag valid modern C# syntax as errors. In particular:
	- **Verify C# syntax claims before posting.** This repository configures `<LangVersion>latest</LangVersion>` centrally (see `Directory.Build.props`). Do not flag valid modern C# syntax as errors. In particular: