Add dedicated test coverage for ExpoPushConnector (#80)

### Summary

Added `tests/Unit/ExpoPushConnectorTest.php`, a dedicated test suite
covering `ExpoPushConnector`'s `resolveBaseUrl()`, `defaultAuth()`,
`defaultHeaders()`, `resolveLimits()`, and `resolveRateLimitStore()`
methods. No production code was changed.

Closes #68

Author: dru1x

tests/Unit/ExpoPushConnectorTest.php

@@ -0,0 +1,107 @@
+<?php
+
+namespace Dru1x\ExpoPush\Tests\Unit;
+
+use Dru1x\ExpoPush\ExpoPushConnector;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use Saloon\Http\Auth\TokenAuthenticator;
+use Saloon\RateLimitPlugin\Exceptions\LimitException;
+use Saloon\RateLimitPlugin\Limit;
+use Saloon\RateLimitPlugin\Stores\MemoryStore;
+
+class ExpoPushConnectorTest extends TestCase
+{
+    #[Test]
+    public function resolves_the_expo_push_api_base_url(): void
+    {
+        $connector = new ExpoPushConnector();
+
+        $this->assertSame('https://exp.host/--/api/v2/push', $connector->resolveBaseUrl());
+    }
+
+    #[Test]
+    public function applies_bearer_token_authentication_when_a_token_is_provided(): void
+    {
+        $connector = new ExpoPushConnector('my-token');
+
+        $authenticator = $connector->getAuthenticator();
+
+        $this->assertInstanceOf(TokenAuthenticator::class, $authenticator);
+        $this->assertSame('my-token', $authenticator->token);
+        $this->assertSame('Bearer', $authenticator->prefix);
+    }
+
+    #[Test]
+    public function applies_no_authentication_when_no_token_is_provided(): void
+    {
+        $connector = new ExpoPushConnector();
+
+        $this->assertNull($connector->getAuthenticator());
+    }
+
+    #[Test]
+    public function default_headers_include_accept_encoding_and_user_agent(): void
+    {
+        $connector = new ExpoPushConnector();
+
+        $this->assertSame('gzip, deflate', $connector->headers()->get('Accept-Encoding'));
+        $this->assertStringStartsWith('expo-server-sdk-php/', (string)$connector->headers()->get('User-Agent'));
+        $this->assertStringContainsString('(dru1x)', (string)$connector->headers()->get('User-Agent'));
+    }
+
+    #[Test]
+    public function default_headers_do_not_leak_an_authorization_header_when_no_token_is_provided(): void
+    {
+        $connector = new ExpoPushConnector();
+
+        $this->assertNull($connector->headers()->get('Authorization'));
+    }
+
+    #[Test]
+    public function rate_limit_allows_six_requests_per_second_and_sleeps_when_exceeded(): void
+    {
+        $connector = new ExpoPushConnector();
+
+        $limit = $this->findLimit($connector, 'expo-push-limit');
+
+        $this->assertSame(6, $limit->getAllow());
+        $this->assertSame(1, $limit->getReleaseInSeconds());
+        $this->assertTrue($limit->getShouldSleep());
+    }
+
+    #[Test]
+    public function uses_a_memory_store_by_default(): void
+    {
+        $connector = new ExpoPushConnector();
+
+        $this->assertInstanceOf(MemoryStore::class, $connector->rateLimitStore());
+    }
+
+    #[Test]
+    public function uses_the_provided_rate_limit_store_when_given(): void
+    {
+        $store = new MemoryStore();
+
+        $connector = new ExpoPushConnector(rateLimitStore: $store);
+
+        $this->assertSame($store, $connector->rateLimitStore());
+    }
+
+    // Internals ----
+
+    private function findLimit(ExpoPushConnector $connector, string $name): Limit
+    {
+        try {
+            foreach ($connector->getLimits() as $limit) {
+                if (str_ends_with($limit->getName(), ":{$name}")) {
+                    return $limit;
+                }
+            }
+        } catch (LimitException $e) {
+            $this->fail($e->getMessage());
+        }
+
+        $this->fail("Limit \"$name\" not found.");
+    }
+}