Add tests to the Connection package to increase coverage. (#39963)

* Migrated PHPUNIT XML file to new schema.

* Migrated schema using phpunit.

* Added test coverage for the Manager signature verification method.

* Minor lint edit.

* Changelog.

* Added an is_null check to make Phan happy.

* Second time's the charm - checking the error object correctly.

---------

Co-authored-by: tbradsha <32492176+tbradsha@users.noreply.github.com>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/12550233129

Upstream-Ref: Automattic/jetpack@59b0e8d

composer.json

@@ -5,7 +5,7 @@
 	"license": "GPL-2.0-or-later",
 	"require": {
 		"php": ">=7.2",
-		"automattic/jetpack-connection": "^6.2.1"
+		"automattic/jetpack-connection": "^6.2.2-alpha"
 	},
 	"require-dev": {
 		"yoast/phpunit-polyfills": "^1.1.1",