Expose source path on LogFileInterface

Add LogFileInterface::getPath(): ?string so detectors can dispatch on a
filename hint when the original path is known. Default implementation on
the abstract LogFile base returns null; PathLogFile records its
constructor argument. StringLogFile and StreamLogFile inherit the null
default. Tests cover both the path and null-fallback cases.

test/tests/Log/File/StringLogFileTest.php

@@ -14,4 +14,11 @@ class StringLogFileTest extends TestCase
 
         $this->assertEquals($content, $logFile->getContent());
     }
+
+    public function testGetPathReturnsNull(): void
+    {
+        $logFile = new StringLogFile("anything");
+
+        $this->assertNull($logFile->getPath());
+    }
 }