Expose the user-defined server_context in instrumentation data

APM integrations need to tag a span with the subject of the request, but the
user-defined `server_context` hash passed to `Server.new` never reaches
`around_request` or `instrumentation_callback`. The exception reporter's
`server_context` argument is a different value — it describes where a failure
occurred (`{ request: ... }`, `{ notification: ... }`), not who made the call —
so there is currently no way to read the request subject from a hook.

Add `Configuration#instrument_server_context`, off by default. When enabled,
`instrument_call` puts the host's `server_context` into the instrumentation data
before invoking `around_request`, so it is readable both before and after
`request_handler.call`.

The flag is opt-in rather than unconditional because the hash is
application-supplied and may hold values a tracing backend should not receive.
Hosts that do not expose a `server_context` reader are unaffected.

`instrument_call` reads it through `self.server_context`: the existing
`server_context:` keyword shadows the reader, and the value wanted here is the
user-defined hash, not the reporter context.

Follows the same shape as the earlier additions of `tool_arguments` (#218) and
`client` (#221) to this hash.

lib/mcp/instrumentation.rb

@@ -7,6 +7,12 @@ module MCP
       begin
         @instrumentation_data = {}
         add_instrumentation_data(method: method)
+        # `self.` is required: the `server_context:` keyword above shadows the
+        # reader, and the value we want here is the user-defined hash passed to
+        # `Server.new`, not the per-call reporter context.
+        if configuration.instrument_server_context? && respond_to?(:server_context)
+          add_instrumentation_data(server_context: self.server_context)
+        end
 
         result = configuration.around_request.call(@instrumentation_data, &block)