fix(drive): answer a runtime unsupported PROPPATCH the way an absent one is

A backend with setProps that rejects unsupported for this path got 501 while a
backend without setProps got 403, for the same inability. 403 is the answer
kept: WebDAV allows it for a property the server will not store, where 501 says
PROPPATCH itself is unimplemented, which is not what the server means.

Refs #179

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/drive/src/server/methods/proppatch.ts

@@ -1,5 +1,6 @@
 import type { Context } from "@webnet/http/server"
 import type { AsyncVFS } from "@webnet/vfs"
+import { isUnsupported } from "@webnet/vfs/fallback"
 import { vfsPathToHref } from "../../common/utils.js"
 import { parse, parseProppatchBody, buildMultistatusXml, makePropKey } from "../xml.js"
 import { el, text } from "@webnet/xml"
@@ -9,17 +10,25 @@ import type { DAVServerOptions } from "../types.js"
 const d = (localName: string) => el(DAV_NS, localName)
 const dt = (localName: string, value: string) => text(DAV_NS, localName, value)
 
+/**
+ * WebDAV lets a server answer 403 for a property it will not store, and a filesystem with no
+ * property storage is that case for every property. 501 would say PROPPATCH itself is
+ * unimplemented, which is not what the server means, and clients read it as a broken server rather
+ * than a limited one. A backend that has `setProps` but rejects `unsupported` for this path is the
+ * same answer rather than a different one, so both arrive here.
+ */
+function cannotStoreProps(ctx: Context): void {
+  ctx.res.setStatus(403, "Forbidden")
+  ctx.res.body = null
+}
+
 export async function handleProppatch(
   ctx: Context,
   vfs: AsyncVFS,
   vfsPath: string,
   opts: DAVServerOptions,
 ): Promise<void> {
-  if (!vfs.setProps) {
-    ctx.res.setStatus(403, "Forbidden")
-    ctx.res.body = null
-    return
-  }
+  if (!vfs.setProps) return cannotStoreProps(ctx)
 
   const bodyText = await ctx.req.text()
   const xml = parse(bodyText)
@@ -35,7 +44,12 @@ export async function handleProppatch(
     delete props[makePropKey(ns, localName)]
   }
 
-  await vfs.setProps(vfsPath, props)
+  try {
+    await vfs.setProps(vfsPath, props)
+  } catch (e) {
+    if (!isUnsupported(e)) throw e
+    return cannotStoreProps(ctx)
+  }
 
   const href = vfsPathToHref(vfsPath, opts.prefix, ctx.req.url)
   const allPropEls = [