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