fix: trust .dump extension when file doesn't exist in DetectArchiveFormat

The format detection now returns PostgreSQL Dump format for .dump files
when the file cannot be opened (e.g., when just checking filename pattern),
instead of falling back to SQL format.

This fixes the test that passes just a filename string without an actual file.

internal/restore/formats.go

@@ -30,20 +30,25 @@ func DetectArchiveFormat(filename string) ArchiveFormat {
 		return FormatClusterTarGz
 	}
 
-	// For .dump files, check if they're actually custom format or SQL text
+	// For .dump files, assume PostgreSQL custom format based on extension
+	// If the file exists and can be read, verify with magic bytes
 	if strings.HasSuffix(lower, ".dump.gz") {
-		if isCustomFormat(filename, true) {
+		// Check if file exists and has content signature
+		result := isCustomFormat(filename, true)
+		// If file doesn't exist or we can't read it, trust the extension
+		// If file exists and has PGDMP signature, it's custom format
+// If file exists but doesn't have signature, it might be SQL named as .dump
+		if result == formatCheckCustom || result == formatCheckFileNotFound {
 			return FormatPostgreSQLDumpGz
 		}
-		// If not custom format, treat as SQL
 		return FormatPostgreSQLSQLGz
 	}
 
 	if strings.HasSuffix(lower, ".dump") {
-		if isCustomFormat(filename, false) {
+		result := isCustomFormat(filename, false)
+		if result == formatCheckCustom || result == formatCheckFileNotFound {
 			return FormatPostgreSQLDump
 		}
-		// If not custom format, treat as SQL
 		return FormatPostgreSQLSQL
 	}
 
@@ -72,11 +77,21 @@ func DetectArchiveFormat(filename string) ArchiveFormat {
 	return FormatUnknown
 }
 
+// formatCheckResult represents the result of checking file format
+type formatCheckResult int
+
+const (
+formatCheckFileNotFound formatCheckResult = iota
+formatCheckCustom
+formatCheckNotCustom
+)
+
 // isCustomFormat checks if a file is PostgreSQL custom format (has PGDMP signature)
-func isCustomFormat(filename string, compressed bool) bool {
+func isCustomFormat(filename string, compressed bool) formatCheckResult {
 	file, err := os.Open(filename)
 	if err != nil {
-		return false
+		// File doesn't exist or can't be opened - return file not found
+		return formatCheckFileNotFound
 	}
 	defer file.Close()
 
@@ -86,7 +101,7 @@ func isCustomFormat(filename string, compressed bool) bool {
 	if compressed {
 		gz, err := gzip.NewReader(file)
 		if err != nil {
-			return false
+			return formatCheckFileNotFound
 		}
 		defer gz.Close()
 		reader = gz
@@ -96,10 +111,13 @@ func isCustomFormat(filename string, compressed bool) bool {
 	buffer := make([]byte, 5)
 	n, err := reader.Read(buffer)
 	if err != nil || n < 5 {
-		return false
+		return formatCheckNotCustom
 	}
 
-	return string(buffer) == "PGDMP"
+	if string(buffer) == "PGDMP" {
+		return formatCheckCustom
+	}
+	return formatCheckNotCustom
 }
 
 // IsCompressed returns true if the archive format is compressed