Add around_request hook for request instrumentation

## Motivation and Context

`instrumentation_callback` fires after request execution, making it impossible to wrap
requests with Application Performance Monitoring (APM) spans (e.g. Datadog tracing).
This adds `around_request`, which wraps request handling and allows executing code
before and after each request.

For example, to wrap requests with a Datadog trace:

```ruby
MCP.configure do |config|
  config.around_request = ->(data, &request_handler) {
    Datadog::Tracing.trace("mcp.#{data[:method]}") do
      request_handler.call
    end
  }
end
```

`instrumentation_callback` is soft-deprecated in favor of `around_request`.

## How Has This Been Tested?

Added tests for `around_request` in configuration, instrumentation,
and server test files. All existing tests continue to pass.

## Breaking Changes

`around_request` itself is optional and defaults to a pass-through,
and `instrumentation_callback` continues to work as before.

However, `instrument_call` now accepts a `server_context:` keyword and its block is invoked
without arguments (previously `yield block` passed the block Proc as an argument).
This affects anyone mixing `MCP::Instrumentation` into their own classes,
though the public `Server` API is unchanged.

Resolves #302.

lib/mcp/instrumentation.rb

@@ -2,19 +2,40 @@
 
 module MCP
   module Instrumentation
-    def instrument_call(method, &block)
+    def instrument_call(method, server_context: {}, exception_already_reported: nil, &block)
       start_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
       begin
         @instrumentation_data = {}
         add_instrumentation_data(method: method)
 
-        result = yield block
+        result = configuration.around_request.call(@instrumentation_data, &block)
 
         result
+      rescue => e
+        already_reported = begin
+          !!exception_already_reported&.call(e)
+        # rubocop:disable Lint/RescueException
+        rescue Exception
+          # rubocop:enable Lint/RescueException
+          # The predicate is expected to be side-effect-free and return a boolean.
+          # Any exception raised from it (including non-StandardError such as SystemExit)
+          # must not shadow the original exception.
+          false
+        end
+
+        unless already_reported
+          add_instrumentation_data(error: :internal_error) unless @instrumentation_data.key?(:error)
+          configuration.exception_reporter.call(e, server_context)
+        end
+
+        raise
       ensure
         end_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
         add_instrumentation_data(duration: end_time - start_time)
 
+        # Backward compatibility: `instrumentation_callback` is soft-deprecated
+        # in favor of `around_request`, but existing callers still expect it
+        # to fire after every request until it is removed in a future version.
         configuration.instrumentation_callback.call(@instrumentation_data)
       end
     end