fileserver: append repeated `hide` subdirectives instead of overwriting (#7817)

Multiple `hide` subdirectives in a file_server Caddyfile block silently overwrote each other, so only the last one took effect. Append to the list instead, so repeated entries accumulate and imported snippets can compose with site-specific hides.

caddytest/integration/caddyfile_adapt/file_server_hide_repeated.caddyfiletest

@@ -0,0 +1,35 @@
+:80
+
+file_server {
+	hide first.txt
+	hide second.txt third.txt
+}
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":80"
+					],
+					"routes": [
+						{
+							"handle": [
+								{
+									"handler": "file_server",
+									"hide": [
+										"first.txt",
+										"second.txt",
+										"third.txt",
+										"./Caddyfile"
+									]
+								}
+							]
+						}
+					]
+				}
+			}
+		}
+	}
+}

modules/caddyhttp/fileserver/caddyfile.go

@@ -91,10 +91,14 @@ func (fsrv *FileServer) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 			fsrv.FileSystem = d.Val()
 
 		case "hide":
-			fsrv.Hide = d.RemainingArgs()
-			if len(fsrv.Hide) == 0 {
+			hide := d.RemainingArgs()
+			if len(hide) == 0 {
 				return d.ArgErr()
 			}
+			// Append so that repeated "hide" subdirectives accumulate
+			// instead of overwriting, which also lets imported snippets
+			// compose with site-specific hides.
+			fsrv.Hide = append(fsrv.Hide, hide...)
 
 		case "index":
 			fsrv.IndexNames = d.RemainingArgs()