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.

test/mcp/configuration_test.rb

@@ -145,6 +145,35 @@ module MCP
       refute merged.validate_tool_call_arguments
     end
 
+    test "defaults instrument_server_context to false" do
+      config = Configuration.new
+
+      refute_predicate config, :instrument_server_context?
+    end
+
+    test "accepts instrument_server_context in the constructor" do
+      config = Configuration.new(instrument_server_context: true)
+
+      assert_predicate config, :instrument_server_context?
+    end
+
+    test "raises ArgumentError when instrument_server_context is not a boolean value" do
+      config = Configuration.new
+
+      exception = assert_raises(ArgumentError) do
+        config.instrument_server_context = "true"
+      end
+      assert_equal("instrument_server_context must be a boolean", exception.message)
+    end
+
+    test "merge carries instrument_server_context from the other configuration" do
+      base = Configuration.new(instrument_server_context: true)
+      other = Configuration.new(instrument_server_context: false)
+
+      refute_predicate base.merge(other), :instrument_server_context?
+      assert_predicate other.merge(base), :instrument_server_context?
+    end
+
     test "defaults validate_tool_call_results to false" do
       config = Configuration.new
       refute config.validate_tool_call_results

test/mcp/instrumentation_test.rb

@@ -40,6 +40,79 @@ module MCP
       end
     end
 
+    # Mirrors `MCP::Server`, which exposes the user-defined hash through
+    # `attr_accessor :server_context`.
+    class SubjectWithServerContext < Subject
+      attr_accessor :server_context
+
+      def initialize(server_context)
+        super()
+        @server_context = server_context
+      end
+    end
+
+    test "#instrument_call omits the user-defined server_context by default" do
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+
+      subject.instrumented_method
+
+      refute_includes subject.instrumentation_data_received.keys, :server_context
+    end
+
+    test "#instrument_call exposes the user-defined server_context when instrument_server_context is enabled" do
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method
+
+      assert_equal({ user_id: 42 }, subject.instrumentation_data_received[:server_context])
+    end
+
+    test "#instrument_call exposes the user-defined server_context to around_request" do
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+      subject.configuration.instrument_server_context = true
+      seen = nil
+      subject.configuration.around_request = ->(data, &handler) {
+        seen = data[:server_context]
+        handler.call
+      }
+
+      subject.instrumented_method
+
+      assert_equal({ user_id: 42 }, seen)
+    end
+
+    test "#instrument_call reports the user-defined server_context as nil when the host does not set one" do
+      subject = SubjectWithServerContext.new(nil)
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method
+
+      assert_includes subject.instrumentation_data_received.keys, :server_context
+      assert_nil subject.instrumentation_data_received[:server_context]
+    end
+
+    test "#instrument_call does not confuse the reporter context with the user-defined server_context" do
+      # `instrument_call(server_context:)` is the exception-reporter context, a
+      # different value from the user-defined hash. Enabling the flag must
+      # surface the latter, not the former.
+      subject = SubjectWithServerContext.new({ user_id: 42 })
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method_with_server_context({ request: "reporter-context" })
+
+      assert_equal({ user_id: 42 }, subject.instrumentation_data_received[:server_context])
+    end
+
+    test "#instrument_call skips server_context for hosts without the reader even when enabled" do
+      subject = Subject.new
+      subject.configuration.instrument_server_context = true
+
+      subject.instrumented_method
+
+      refute_includes subject.instrumentation_data_received.keys, :server_context
+    end
+
     test "#instrument_call adds the method name to the instrumentation data" do
       subject = Subject.new