fix(streamable-http): gracefully shutdown while client connected

[greenhat616]

Close #493

I’m adding a cancellation token to Streamable HTTP to support graceful shutdowns. Because I’m unclear on the MCP protocol’s semantics for initiating termination, I’m currently just cutting off the body stream.

Motivation and Context

In desktop apps, we need a mechanism for the process to proactively terminate (e.g., for updates or user-initiated quits). Today, the MCP Client holds the connection open, which effectively prevents the app from shutting down; most users aren’t aware this is intentional client behavior.

How Has This Been Tested?

Breaking Changes

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

[greenhat616]

I checked the doc: https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle#shutdown

It seems that trigger a connection close is enough

[Copilot]

Pull Request Overview

This PR adds cancellation token support to the StreamableHttpServer to enable graceful shutdown of SSE streams. When a server shutdown is initiated (e.g., via Ctrl+C), the cancellation token ensures that all active SSE connections are properly terminated.

• Added cancellation_token field to StreamableHttpServerConfig
• Updated sse_stream_response to accept and use a cancellation token to terminate streams
• Modified the example to demonstrate graceful shutdown with cancellation token integration

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
examples/servers/src/counter_streamhttp.rs	Updated imports, instantiated cancellation token, configured it in StreamableHttpServerConfig, and integrated cancellation into graceful shutdown handler
crates/rmcp/src/transport/streamable_http_server/tower.rs	Added cancellation_token field to StreamableHttpServerConfig and passed child tokens to all sse_stream_response calls
crates/rmcp/src/transport/common/server_side_http.rs	Updated sse_stream_response function to accept cancellation token and use it to terminate SSE streams via take_until

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