remote server: Fix error log about inability to open buffer (#19824)

Turns out that we used client-side `fs` to check whether something is a
directory or not, which obviously doesn't work with SSH projects.

Release Notes:

- N/A

---------

Co-authored-by: Bennet <bennet@zed.dev>

crates/proto/proto/zed.proto

@@ -259,9 +259,6 @@ message Envelope {
         CloseBuffer close_buffer = 245;
         UpdateUserSettings update_user_settings = 246;
 
-        CheckFileExists check_file_exists = 255;
-        CheckFileExistsResponse check_file_exists_response = 256;
-
         ShutdownRemoteServer shutdown_remote_server = 257;
 
         RemoveWorktree remove_worktree = 258;
@@ -284,13 +281,16 @@ message Envelope {
         GitBranchesResponse git_branches_response = 271;
 
         UpdateGitBranch update_git_branch = 272;
+
         ListToolchains list_toolchains = 273;
         ListToolchainsResponse list_toolchains_response = 274;
         ActivateToolchain activate_toolchain = 275;
         ActiveToolchain active_toolchain = 276;
-        ActiveToolchainResponse active_toolchain_response = 277; // current max
-    }
+        ActiveToolchainResponse active_toolchain_response = 277;
 
+        GetPathMetadata get_path_metadata = 278;
+        GetPathMetadataResponse get_path_metadata_response = 279; // current max
+    }
 
     reserved 87 to 88;
     reserved 158 to 161;
@@ -305,6 +305,7 @@ message Envelope {
     reserved 221;
     reserved 224 to 229;
     reserved 247 to 254;
+    reserved 255 to 256;
 }
 
 // Messages
@@ -2357,14 +2358,15 @@ message UpdateUserSettings {
     }
 }
 
-message CheckFileExists {
+message GetPathMetadata {
     uint64 project_id = 1;
     string path = 2;
 }
 
-message CheckFileExistsResponse {
+message GetPathMetadataResponse {
     bool exists = 1;
     string path = 2;
+    bool is_dir = 3;
 }
 
 message ShutdownRemoteServer {}

crates/proto/src/proto.rs

@@ -343,8 +343,6 @@ messages!(
     (FindSearchCandidatesResponse, Background),
     (CloseBuffer, Foreground),
     (UpdateUserSettings, Foreground),
-    (CheckFileExists, Background),
-    (CheckFileExistsResponse, Background),
     (ShutdownRemoteServer, Foreground),
     (RemoveWorktree, Foreground),
     (LanguageServerLog, Foreground),
@@ -363,7 +361,9 @@ messages!(
     (ListToolchainsResponse, Foreground),
     (ActivateToolchain, Foreground),
     (ActiveToolchain, Foreground),
-    (ActiveToolchainResponse, Foreground)
+    (ActiveToolchainResponse, Foreground),
+    (GetPathMetadata, Background),
+    (GetPathMetadataResponse, Background)
 );
 
 request_messages!(
@@ -472,7 +472,6 @@ request_messages!(
     (SynchronizeContexts, SynchronizeContextsResponse),
     (LspExtSwitchSourceHeader, LspExtSwitchSourceHeaderResponse),
     (AddWorktree, AddWorktreeResponse),
-    (CheckFileExists, CheckFileExistsResponse),
     (ShutdownRemoteServer, Ack),
     (RemoveWorktree, Ack),
     (OpenServerSettings, OpenBufferResponse),
@@ -483,7 +482,8 @@ request_messages!(
     (UpdateGitBranch, Ack),
     (ListToolchains, ListToolchainsResponse),
     (ActivateToolchain, Ack),
-    (ActiveToolchain, ActiveToolchainResponse)
+    (ActiveToolchain, ActiveToolchainResponse),
+    (GetPathMetadata, GetPathMetadataResponse)
 );
 
 entity_messages!(
@@ -555,7 +555,6 @@ entity_messages!(
     SynchronizeContexts,
     LspExtSwitchSourceHeader,
     UpdateUserSettings,
-    CheckFileExists,
     LanguageServerLog,
     Toast,
     HideToast,
@@ -566,7 +565,8 @@ entity_messages!(
     UpdateGitBranch,
     ListToolchains,
     ActivateToolchain,
-    ActiveToolchain
+    ActiveToolchain,
+    GetPathMetadata
 );
 
 entity_messages!(