Leave listChanged out of initialize when notifications cannot be delivered

[koic]

Motivation and Context

initialize answers with the server's capabilities as configured, by default listChanged: true on tools, prompts and resources, whatever the transport can deliver. Stateless Streamable HTTP has no session streams, so send_notification reports non-delivery there and a notifications/*/list_changed never reaches a handshake client, yet initialize promised it (issue #619). The same server already leaves the flags out of server/discover when subscriptions/listen is not served, so the two lifecycles described it differently. The specification defines listChanged as whether the server will emit the notification when the list changes.

Transport#delivers_notifications? now tells the server whether notifications reach a client outside a response (true by default, false for stateless Streamable HTTP), and initialize leaves listChanged and subscribe out when they do not, with the stripping server/discover already applies. The capability objects themselves stay: prompts and resources with nothing registered still answer their list methods, as the Python SDK advertises them too, and logging stays on both lifecycles, since logging/setLevel is served, although stateless Streamable HTTP drops a handshake client's log messages, as its documentation says.

Each lifecycle keeps the flags its own delivery path can honor, so the two still differ where those paths do: a stateless transport serving subscriptions/listen keeps them in server/discover, since listen streams deliver the notifications there, but not in initialize, and a stateful one without it does the reverse.

Fixes #619.

How Has This Been Tested?

test/mcp/server_test.rb (the flags are stripped when the transport delivers no notifications, and kept when it does or does not say) and test/mcp/server/transports/streamable_http_transport_test.rb (a stateless transport's initialize advertises no flags and, without subscriptions/listen, the same capabilities as its server/discover; a stateful one keeps them). Against the previous library the stripping tests fail on the present flags.
The same files also cover a Capabilities builder, string keys, the configured capabilities staying unchanged, and the two configurations where only one lifecycle keeps the flags.

Breaking Changes

The initialize result of a stateless Streamable HTTP server carries no listChanged or subscribe flags.

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