feat: add subscription listen streams (SEP-2575)

[DaleSeo]

Motivation and Context

SEP-2575 replaces the old way of delivering notifications through resources/subscribe, resources/unsubscribe, and standalone Streamable HTTP GET streams. It introduces a new transport-neutral request called subscriptions/listen. This update brings in typed models and high-level server/client APIs that ensure acknowledgment ordering, subscription ID tagging, opt-in filtering, concurrent stream isolation, cancellation, and smooth or abrupt termination for both stdio and Streamable HTTP.

We still support legacy resource subscriptions, HTTP sessions, GET/DELETE, and resumable streams for older protocol versions.

How Has This Been Tested?

Add tests. The conformance suite passes

Breaking Changes

No existing stable-protocol behavior is removed. The legacy subscribe() and unsubscribe() APIs are deprecated for the 2026 protocol revision but retain their existing wire behavior for older negotiated versions.

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