fix: stamp server info on graceful subscription results

[mrcs64]

Stamp the server implementation identity into the final result produced when a
subscriptions/listen handler completes gracefully.

Motivation and Context

The MCP 2026-07-28 specification says servers SHOULD include
io.modelcontextprotocol/serverInfo in every result's _meta.
ServerHandler::listen returns Result<(), McpError>, and rmcp constructs the
final SubscriptionsListenResult only after the handler returns. Applications
therefore cannot add this metadata themselves.

This PR intentionally covers only the graceful subscriptions/listen result
synthesized by rmcp after the application handler returns. SDK-wide stamping of
application-produced results is a separate concern.

How Has This Been Tested?

• Added a stdio regression test that asserts the graceful final result contains
  the server implementation identity.
• Added the same assertion to the real Streamable HTTP/SSE graceful-close test.
• Ran the full repository test recipe, including the all-features and
  non-local-feature passes.
• Ran cargo clippy --all-targets --all-features -- -D warnings.
• Ran cargo +nightly fmt --all -- --check.

Breaking Changes

None. This adds missing response metadata without changing the handler API.

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

The request path captures ServerInfo once before establishing the
subscription, so capability filtering and the eventual server identity come
from the same snapshot. Cancellation, abrupt transport closure, and handler
errors keep their existing behavior.

[DaleSeo]

Thanks for the fix, @mrcs64!

[DaleSeo]

Just for context, I initially implemented the shape documented in SEP-2575 in #1000. However, the final spec added a graceful result for subscriptions/listen and made serverInfo optional in the result metadata. Since the SEP page was not updated, those changes were easy to miss, so I opened modelcontextprotocol/modelcontextprotocol#3159 to document these post-final SEP changes for future implementers.