Require php-http/mock-client 1.5 or newer for the tests

With "php-http/mock-client": "^1.3", the lowest-dependency build resolves
mock-client 1.3.0 together with php-http/httplug 1.1.0, whose HttpClient
interface predates PSR-18, so Http\Mock\Client is not a
Psr\Http\Client\ClientInterface and every test fails with a TypeError.
The "php-http/httplug": "^2.0" entry that used to pin HTTPlug 2 was
removed as unused in #9. mock-client 1.5.0 and newer require httplug
^2.0 themselves, so raising the lower bound restores a PSR-18 mock
client in the lowest build; the highest build keeps mock-client 1.6.1.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: claude

composer.json

@@ -17,7 +17,7 @@
     "require-dev": {
         "nyholm/psr7": "^1.2",
         "php-http/discovery": "^1.7",
-        "php-http/mock-client": "^1.3",
+        "php-http/mock-client": "^1.5",
         "phpunit/phpunit": "^8.5"
     },
     "autoload": {