Extract parallel directory-URI validator in Skills handlers

_handle_read_directory inlined the same "validate incoming URI, convert
ValueError to MCPError" pattern that _handle_get had already extracted
into a helper. Add a parallel _require_directory_uri so both handlers
open with a symmetric one-line precondition check, matching the
_require_ui_scheme helper idiom from the Apps extension.

Author: vijay

src/mcp/server/skills.py

@@ -126,10 +126,7 @@ async def _handle_read_directory(
         self, ctx: ServerRequestContext[Any, Any], params: ReadDirectoryParams
     ) -> HandlerResult:
         assert self._read_directory is not None
-        try:
-            parse_directory_uri(params.uri)
-        except ValueError as exc:
-            raise MCPError(code=INVALID_PARAMS, message=str(exc)) from exc
+        _require_directory_uri(params.uri)
         result = await self._read_directory(ctx, params)
         try:
             validate_directory_result(params.uri, result)
@@ -147,6 +144,13 @@ def _require_skill_md_uri(uri: str) -> None:
         raise MCPError(code=INVALID_PARAMS, message=str(exc)) from exc
 
 
+def _require_directory_uri(uri: str) -> None:
+    try:
+        parse_directory_uri(uri)
+    except ValueError as exc:
+        raise MCPError(code=INVALID_PARAMS, message=str(exc)) from exc
+
+
 def _finalize_cacheable(result: ListSkillsResult, protocol_version: str) -> HandlerResult:
     """Gate SEP-2549's `ttlMs`/`cacheScope` to protocol version 2026-07-28+.