Improved tag conflict docs and tests

docs/swagger/paths/v1_tags.json

@@ -229,6 +229,16 @@
                     }
                 }
             },
+            "409": {
+                "description": "The name provided in newName param is already in use for another tag.",
+                "content": {
+                    "application/problem+json": {
+                        "schema": {
+                            "$ref": "../definitions/Error.json"
+                        }
+                    }
+                }
+            },
             "500": {
                 "description": "Unexpected error.",
                 "content": {

module/Core/test/Exception/TagConflictExceptionTest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Rest\Exception;
+
+use PHPUnit\Framework\TestCase;
+use Shlinkio\Shlink\Core\Exception\TagConflictException;
+
+use function sprintf;
+
+class TagConflictExceptionTest extends TestCase
+{
+    /** @test */
+    public function properlyCreatesExceptionFromNotFoundTag(): void
+    {
+        $oldName = 'foo';
+        $newName = 'bar';
+        $expectedMessage = sprintf('You cannot rename tag %s to %s, because it already exists', $oldName, $newName);
+        $e = TagConflictException::fromExistingTag($oldName, $newName);
+
+        $this->assertEquals($expectedMessage, $e->getMessage());
+        $this->assertEquals($expectedMessage, $e->getDetail());
+        $this->assertEquals('Tag conflict', $e->getTitle());
+        $this->assertEquals('TAG_CONFLICT', $e->getType());
+        $this->assertEquals(['oldName' => $oldName, 'newName' => $newName], $e->getAdditionalData());
+        $this->assertEquals(409, $e->getStatus());
+    }
+}