Infer Windows UNC paths with forward and mixed slashes (#49424)

## Why

`LegacyAppPathString` treated `//server/share/project` as POSIX, losing its UNC interpretation when inferring a `PathUri`.

## What changed

Infer Windows syntax whenever an API path starts with two separators, including forward and mixed slashes, independently of the host. For example, `//server/share/project` now converts to `file://server/share/project`. Double-slash POSIX paths require an explicit `PathConvention::Posix`; inferred paths still undergo validation.

## Testing

Add regression tests for UNC and namespace paths, mixed separators, URI serialization round trips, explicit POSIX conversion, convention mismatches, and malformed UNC paths.

GitOrigin-RevId: 720893de517addb7a938e6784d1ed0f05df3ec0d

codex-rs/utils/path-uri/src/api_path_string_tests.rs

@@ -418,7 +418,7 @@ fn infers_absolute_path_conventions_from_api_text() {
         (r"\\.\COM1", Some(PathConvention::Windows)),
         ("/workspace/file.rs", Some(PathConvention::Posix)),
         ("/C:/workspace/file.rs", Some(PathConvention::Posix)),
-        ("//server/share/file.rs", Some(PathConvention::Posix)),
+        ("//server/share/file.rs", Some(PathConvention::Windows)),
         ("", None),
         (".", None),
         ("subdir/file.rs", None),
@@ -657,3 +657,62 @@ fn serializes_and_deserializes_as_a_string() {
         rendered
     );
 }
+
+#[test]
+fn inferred_unc_paths_accept_forward_and_mixed_slashes_on_every_host() {
+    for raw in [
+        "//server/share/project",
+        r"//server/share\project",
+        r"/\server/share/project",
+        r"\/server/share/project",
+        r"\\server\share\project",
+        "//wsl.localhost/Ubuntu/home/user/project",
+        "//127.0.0.1/c$/project",
+        "//LOCALHOST/c$/project",
+        "//?/C:/project",
+    ] {
+        let path = LegacyAppPathString::from_string(raw);
+        let expected = path.to_path_uri(PathConvention::Windows).expect(raw);
+        let inferred = path.to_inferred_path_uri().expect(raw);
+        assert_eq!(
+            (inferred.clone(), inferred.infer_path_convention()),
+            (expected, Some(PathConvention::Windows)),
+            "{raw}"
+        );
+        let serialized = serde_json::to_string(&inferred).expect("serialize URI");
+        assert_eq!(
+            serde_json::from_str::<PathUri>(&serialized).expect("deserialize URI"),
+            inferred,
+            "{raw}"
+        );
+    }
+    let slash = LegacyAppPathString::from_string("//server/share/project")
+        .to_inferred_path_uri()
+        .unwrap();
+    assert_eq!(
+        slash,
+        PathUri::parse("file://server/share/project").unwrap()
+    );
+}
+
+#[test]
+fn double_slash_posix_paths_require_an_explicit_convention() {
+    let path = LegacyAppPathString::from_string("//server/share/project");
+    assert_eq!(
+        path.to_path_uri(PathConvention::Posix).unwrap(),
+        PathUri::parse("file:///server/share/project").unwrap()
+    );
+    let cwd = PathUri::parse("file:///workspace").unwrap();
+    assert!(matches!(
+        path.resolve_against(&cwd, /*user_home_dir*/ None),
+        Err(LegacyAppPathStringError::MismatchedConvention { .. })
+    ));
+    for raw in ["//", "//server", "//server/", "///server/share"] {
+        assert!(
+            LegacyAppPathString::from_string(raw)
+                .to_inferred_path_uri()
+                .is_none(),
+            "{raw}"
+        );
+    }
+}