Add server-side Streamable HTTP transport support

[halter73]

Adds server-side Streamable HTTP transport support. Client-side support will come in a later PR.

• Changes IJsonRpcMessage to an abstract base class so RelatedTransport will always be available
• Streamable HTTP supports multiple concurrent HTTP request with their own independent SSE response streams
• RelatedTransport indicates the source or destination of the JsonRpcMessage
• Changes the default RequestId to a JSON number for better compatibility with MCP servers in the wild

Contributes to #157
Contributes to #158

[Copilot]

Pull Request Overview

This pull request introduces a Streamable HTTP transport for the MCP server with updates to message type handling and session management. Key changes include refactoring JSON-RPC message types from interface/record to abstract classes, updating HTTP endpoint implementations, and adding an idle session background service for cleanup.

Reviewed Changes

Copilot reviewed 44 out of 44 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Protocol/Transport/SseResponseStreamTransport.cs	Updated message types and removed legacy JSON writer helper.
Protocol/Transport/SseClientSessionTransport.cs	Replaced interface types with concrete message types for consistency.
Protocol/Messages/*	Converted several records and interfaces into classes/abstract classes with unified JSON-RPC properties.
AspNetCore/StreamableHttpHandler.cs	Implements Streamable HTTP transport with session management and updated error handling.
AspNetCore/*	Various endpoint and session management updates, including idle session cleanup in background service.

Comments suppressed due to low confidence (2)

src/ModelContextProtocol/AspNetCore/StreamableHttpHandler.cs:80

• The GET endpoint should verify the presence of 'text/event-stream' in the Accept header rather than 'application/json', since the response is expected to be an SSE stream. Update the check to use 'text/event-stream' to correctly validate client support.

if (!acceptHeader.Contains("application/json", StringComparison.Ordinal))

src/ModelContextProtocol/AspNetCore/HttpServerTransportOptions.cs:30

• There is a duplicate word 'minutes' in the documentation comment. Remove the redundant 'minutes' for clarity.

/// minutes.