fix: correct MISSING_REQUIRED_CLIENT_CAPABILITY error code to -32021

[MyNameIsMeerkat]

Updates the Missing Required Client Capability error code from -32003 to -32021 to match the shipped core specification.

Motivation and Context

Core renumbered this error code as part of the 2026-07-28 release and corrected its own copy of SEP-2663 in commit 9b44c6b ("fix: correct MISSING_REQUIRED_CLIENT_CAPABILITY error codes in SEP-2663"). The conformance suite followed and now asserts -32021.

This repository did not seem to renumber. Both specification/draft/tasks.md and its copy of SEP-2663 still specify -32003 throughout their text.

The practical effect is that the conformance suite currently tests for an error code the extension's normative text does not define. A server author who implements this specification literally will emit -32003 for a non-declaring client and fail conformance, with the spec on their side and the test suite against them.

How Has This Been Tested?

NA. The change is to error code literals in normative prose + JSON examples. No schema changes. The resulting code matches what the conformance suite already asserts.

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

None