fix(language_server): don't resend diagnostic on save, when typeAware is disabled and run is onType (#13604)

Fixes an edge case introduced in #13332 where diagnostics disappear when
saving a file with `oxc.lint.run: "onType"` and `oxc.typeAware: false`.

## Problem

The logic from #13332 assumes `tsgolint` always exists when
`Run::OnType`:
```rust
(ServerLinterRun::OnSave, Run::OnType) => (false, true)  // always run tsgolint
```

But when `type_aware: false`, there is no `tsgo_linter`, causing the
server to return empty diagnostics instead of None, which clears all
error highlights in VSCode.



![clear-diagnostics](https://github.com/user-attachments/assets/3f7a530e-5319-4fa5-8668-7a34a3742e82)

## Solution

Check if `tsgo_linter` actually exists:
```rust
(ServerLinterRun::OnSave, Run::OnType) => {
    let should_run_tsgo = self.tsgo_linter.as_ref().is_some();
    (false, should_run_tsgo)
}
```

Now returns None when no linters should run, preserving existing
diagnostics.

**Test:** Added `test_lint_on_run_on_type_on_save_without_type_aware`
with fixture

Author: yongsk0066

crates/oxc_language_server/fixtures/linter/lint_on_run/on_type/on-save-no-type-aware.ts

@@ -0,0 +1,7 @@
+debugger;
+
+async function returnsPromise() {
+  return "value";
+}
+
+returnsPromise().then(() => {});

crates/oxc_language_server/src/linter/server_linter.rs

@@ -331,9 +331,12 @@ impl ServerLinter {
             (ServerLinterRun::OnType, Run::OnType) => (true, false),
             // it does not match, run nothing
             (ServerLinterRun::OnType, Run::OnSave) => (false, false),
-            // run only tsglint on save, even if the user wants it with type
-            // tsgolint only supports the OS file system.
-            (ServerLinterRun::OnSave, Run::OnType) => (false, true),
+            // In onType mode, only TypeScript type checking runs on save
+            // If type_aware is disabled (tsgo_linter is None), skip everything to preserve diagnostics
+            (ServerLinterRun::OnSave, Run::OnType) => {
+                let should_run_tsgo = self.tsgo_linter.as_ref().is_some();
+                (false, should_run_tsgo)
+            }
         };
 
         // return `None` when both tools do not want to be used
@@ -483,6 +486,16 @@ mod test {
         .test_and_snapshot_single_file_with_run_type("on-save.ts", Run::OnSave);
     }
 
+    #[test]
+    #[cfg(not(target_endian = "big"))]
+    fn test_lint_on_run_on_type_on_save_without_type_aware() {
+        Tester::new(
+            "fixtures/linter/lint_on_run/on_type",
+            Some(Options { type_aware: false, run: Run::OnType, ..Default::default() }),
+        )
+        .test_and_snapshot_single_file_with_run_type("on-save-no-type-aware.ts", Run::OnSave);
+    }
+
     #[test]
     fn test_no_errors() {
         Tester::new("fixtures/linter/no_errors", None)

crates/oxc_language_server/src/snapshots/fixtures_linter_lint_on_run_on_type@on-save-no-type-aware.ts.snap

@@ -0,0 +1,5 @@
+---
+source: crates/oxc_language_server/src/tester.rs
+input_file: crates/oxc_language_server/fixtures/linter/lint_on_run/on_type/on-save-no-type-aware.ts
+---
+File is ignored