docs: require a passing conformance test for new 2026-07-28 spec features

[maxisbey]

Adds a bullet to the AGENTS.md Testing section requiring that new features from the 2026-07-28 spec have a matching, passing test in the conformance suite — and that coding agents stop and surface a gap to the user (so it can be raised on the conformance repo) instead of skipping the check silently.

Motivation and Context

As spec features land for the 2026-07-28 version, the conformance suite is how we verify cross-SDK behavior. Without a written rule, agent-driven contributions tend to treat unit tests as sufficient and never check conformance coverage. This makes the conformance check part of the documented definition of done and gives a clear escalation path (file an issue on modelcontextprotocol/conformance) when the suite has no scenario for a feature.

How Has This Been Tested?

Docs-only change; markdownlint passes via pre-commit.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

CI already runs the suite both ways (server via the everything-server, client via the conformance client harness) in .github/workflows/conformance.yml; the new bullet points there for the wire-up.