Kotlin extractor: keep synthetic locations for unresolved file classes

Why this is needed:
- library-tests/multiple_files/method_accesses.ql regressed because receiver class locations for external file-class members became concrete file paths.
- For stdlib-style unresolved container-source paths, forcing a concrete location changed stable output from synthetic unknown location to external path-based locations.

What changed:
- Added shouldUseConcreteExternalFileClassLocation to distinguish reliable concrete paths from unresolved placeholders.
- In external package-fragment parent handling, only write an external file-class location when the normalized path is concrete and stable.
- If no reliable path is available, keep prior synthetic behaviour by not forcing a concrete location.

Effect:
- Restores stable receiver-location output for method_accesses while preserving concrete locations when we have trustworthy binary-path information.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

java/kotlin-extractor/src/main/kotlin/KotlinUsesExtractor.kt

@@ -1005,9 +1005,10 @@ open class KotlinUsesExtractor(
                 val binaryPath =
                     getContainerSourceBinaryPath(d.containerSource)
                         ?.let { normalizeExternalFileClassBinaryPath(it, fqName) }
-                        ?: "/!unknown-binary-location/${fqName.asString().replace(".", "/")}.class"
-                val fileId = tw.mkFileId(binaryPath, true)
-                tw.writeHasLocation(fileClassId, tw.getWholeFileLocation(fileId))
+                if (binaryPath != null && shouldUseConcreteExternalFileClassLocation(binaryPath)) {
+                    val fileId = tw.mkFileId(binaryPath, true)
+                    tw.writeHasLocation(fileClassId, tw.getWholeFileLocation(fileId))
+                }
             }
             return fileClassId
         }

java/kotlin-extractor/src/main/kotlin/utils/ClassNames.kt

@@ -228,6 +228,12 @@ fun normalizeExternalFileClassBinaryPath(path: String, fqName: FqName): String {
     return path
 }
 
+fun shouldUseConcreteExternalFileClassLocation(path: String): Boolean {
+    val normalizedPath = path.replace('\\', '/')
+    return normalizedPath.contains("/") &&
+        !normalizedPath.startsWith("/!unknown-binary-location/")
+}
+
 fun getJavaEquivalentClassId(c: IrClass) =
     c.fqNameWhenAvailable?.toUnsafe()?.let { JavaToKotlinClassMap.mapKotlinToJava(it) }