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.

src/Log/File/LogFileInterface.php

@@ -15,4 +15,15 @@ interface LogFileInterface
      * @return string
      */
     public function getContent(): string;
+
+    /**
+     * Get the source path of the log file when one is known
+     *
+     * Returns null for log files without a filesystem origin (string content,
+     * arbitrary streams). Concrete implementations should return the path used
+     * to construct them when applicable.
+     *
+     * @return string|null
+     */
+    public function getPath(): ?string;
 }