Refuse workflows that move cards in a loop

Every enabled workflow is an edge "source stack -> target stack";
LoopDetector finds circles in that graph, a workflow whose source is its
own target being the shortest. Without a check, two workflows A -> B and
B -> A move the same cards back and forth on every run, even within a
single run, with a notice for each move. The trigger date is ignored on
purpose: by start date one way and by due date the other, a card with
both dates passed still circles. Chains stay allowed.

The controller rejects saving an enabled workflow that would close a loop
and names the workflows involved. Workflows stored before this check are
skipped by the runner (logged, not disabled) and flagged in the settings
list via a new `loop` field from index().

tests/Unit/Service/LoopDetectorTest.php

@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\WorkflowDeckAutomation\Tests\Unit\Service;
+
+use OCA\WorkflowDeckAutomation\Service\LoopDetector;
+use PHPUnit\Framework\TestCase;
+
+class LoopDetectorTest extends TestCase {
+	public function testNoWorkflowsHaveNoLoop(): void {
+		$this->assertSame([], LoopDetector::findLoopingWorkflows([]));
+	}
+
+	public function testSameSourceAndTargetIsALoop(): void {
+		$this->assertSame([1], LoopDetector::findLoopingWorkflows([1 => [10, 10]]));
+	}
+
+	public function testTwoWorkflowsBackAndForthAreALoop(): void {
+		$this->assertSame([1, 2], LoopDetector::findLoopingWorkflows([
+			1 => [10, 20],
+			2 => [20, 10],
+		]));
+	}
+
+	public function testLongerCircleIsALoop(): void {
+		$this->assertSame([1, 2, 3], LoopDetector::findLoopingWorkflows([
+			1 => [10, 20],
+			2 => [20, 30],
+			3 => [30, 10],
+		]));
+	}
+
+	public function testChainIsNoLoop(): void {
+		$this->assertSame([], LoopDetector::findLoopingWorkflows([
+			1 => [10, 20],
+			2 => [20, 30],
+		]));
+	}
+
+	public function testOnlyWorkflowsOnTheCircleAreReported(): void {
+		// 1 feeds into the circle 2 → 3 → 2 and 4 leads out of it; neither
+		// lies on it.
+		$this->assertSame([2, 3], LoopDetector::findLoopingWorkflows([
+			1 => [10, 20],
+			2 => [20, 30],
+			3 => [30, 20],
+			4 => [30, 40],
+		]));
+	}
+
+	public function testTwoWorkflowsFromTheSameSourceAreNoLoop(): void {
+		$this->assertSame([], LoopDetector::findLoopingWorkflows([
+			1 => [10, 20],
+			2 => [10, 30],
+		]));
+	}
+
+	public function testFindPathListsTheWorkflowsInOrder(): void {
+		$edges = [
+			7 => [30, 10],
+			5 => [10, 20],
+			6 => [20, 30],
+		];
+		$this->assertSame([5, 6], LoopDetector::findPath($edges, 10, 30));
+		$this->assertSame([6, 7], LoopDetector::findPath($edges, 20, 10));
+	}
+
+	public function testFindPathWithoutConnectionIsNull(): void {
+		$this->assertNull(LoopDetector::findPath([1 => [10, 20]], 20, 10));
+	}
+
+	public function testFindPathToItselfIsEmpty(): void {
+		$this->assertSame([], LoopDetector::findPath([], 10, 10));
+	}
+}