[Tests] Introduce client-server integration tests (#419)

* [Tests] Introduce client-server integration tests

* Add an in-process loopback that connects a real client transport to a real
  server transport, draining both Fiber loops in one synchronous pass instead
  of polling. A stalled exchange fails where it happened rather than hanging.
* Cover protocol version negotiation across both implementations, where the
  unit tests only ever drive one side against a canned counter-offer.
* Cover the server-to-client round-trips end to end: elicitation, sampling,
  roots, plus progress and logging notifications.
* Add the `integration` test suite and a `make integration-tests` target.

* [Tests] Run the integration suite in CI

The unit job pins `--testsuite=unit`, so the new suite would never have run on
GitHub. It goes in that job rather than its own: the loopback leans on Fibers,
and the matrix is what covers them down to the PHP 8.1 floor.

* [Tests] Run integration tests against a real server process

Replaces the in-process loopback with the setup from examples/client: the
client spawns a fixture server over the real StdioTransport, so the tests
exercise the shipped transports and no longer nest both sides' Fibers.

Moves the suite into its own CI job, matrixed over PHP 8.1-8.5.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: deps-stable deps-low cs phpstan tests unit-tests inspector-tests coverage ci ci-stable ci-lowest conformance-tests conformance-server conformance-client docs
+.PHONY: deps-stable deps-low cs phpstan tests unit-tests integration-tests inspector-tests coverage ci ci-stable ci-lowest conformance-tests conformance-server conformance-client docs
 
 deps-stable:
 	composer update --prefer-stable
@@ -18,6 +18,9 @@ tests:
 unit-tests:
 	vendor/bin/phpunit --testsuite=unit
 
+integration-tests:
+	vendor/bin/phpunit --testsuite=integration
+
 inspector-tests:
 	vendor/bin/phpunit --testsuite=inspector