## Motivation and Context
The MCP specification defines `ping` as a bidirectional utility: either party may issue it.
https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/ping
The previous PR added `MCP::Client#ping`. This PR adds the symmetric server-side send capability,
`ServerSession#ping`.
This aligns the Ruby SDK with the Python SDK (`ServerSession.send_ping`) and
the TypeScript SDK (`Server.ping()`), both of which expose a server-initiated ping.
In practice this enables a Ruby MCP server to health-check non-Ruby clients
(e.g., Claude Desktop, the Python or TypeScript SDK client) during long-running tool operations.
Scope is intentionally server-only: this PR does not add client-side inbound request dispatch,
which is a larger refactor outside the ping utility. A Ruby `MCP::Client` therefore still cannot receive
a server-initiated ping; that gap is tracked as a separate follow-up.
## How Has This Been Tested?
New file `test/mcp/server_session_ping_test.rb` covers:
- success (empty `result` returned)
- `related_request_id` propagated through the transport when a session ID is set
- `ValidationError` when the transport returns `nil`
- `ValidationError` when the transport returns a non-Hash `result`
- transport-level errors propagate unchanged
- ping succeeds without any client capability declaration (per spec, ping has no capability gate)
`test/mcp/server_context_test.rb` adds two tests for `ServerContext#ping` delegation
(happy path and `NoMethodError` when the session lacks `#ping`).
The existing server-side `ping` handler tests continue to pass unchanged.
## Breaking Changes
None. `MCP::ServerContext#ping` and `MCP::Server::ValidationError` are purely additive.
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