fix(reference): resolve extensionless imports to .mts and .cts files (#4223)

* fix(reference): resolve extensionless imports to .mts and .cts files

Include .mts and .cts extension probe slots in RESOLUTION_SUFFIXES so that relative imports and requires targeting ESM and CJS TypeScript files without an explicit extension resolve to internal relationships instead of remaining unresolved.

* test(reference): restore regression coverage for extensionless .mts and .cts imports

Restore tests covering extensionless imports, requires, and directory indexes to .mts and .cts files so that probe suffix omissions are caught by automated test regression.

src/application/ReferenceSourceImportEntries.test.ts

@@ -253,3 +253,67 @@ it("matches package manifests by exact or unique canonical directory identity",
     `${decomposedTwin}/exact-decomposed.js`,
   ]);
 });
+
+it("resolves extensionless imports and requires to .mts and .cts files and directory indexes", () => {
+  const files = [
+    [
+      "src/index.ts",
+      'import "./esm";\nimport "./pkg-esm";\nimport cjs = require("./cjs");\nimport pkgCjs = require("./pkg-cjs");\n',
+    ],
+    ["src/esm.mts", "export const esm = 1;\n"],
+    ["src/pkg-esm/index.mts", "export const pkgEsm = 2;\n"],
+    ["src/cjs.cts", "module.exports = { cjs: 3 };\n"],
+    ["src/pkg-cjs/index.cts", "module.exports = { pkgCjs: 4 };\n"],
+  ] as const;
+  const read: ReferenceSourceRead = {
+    root: "/selected/source",
+    entries: files.map(([path, content]) => ({
+      status: "read",
+      kind: "file",
+      path,
+      bytes: Buffer.from(content),
+      size: Buffer.byteLength(content),
+    })),
+    bytesRead: files.reduce(
+      (sum, [, content]) => sum + Buffer.byteLength(content),
+      0,
+    ),
+    limitations: [],
+  };
+  const parsed = parseReferenceSourceEntries(
+    read,
+    new Set(files.map(([path]) => path)),
+  );
+  expect(parsed.relationships).toEqual(
+    expect.arrayContaining([
+      {
+        from_path: "src/index.ts",
+        to: "src/esm.mts",
+        kind: "imports",
+        resolution: "internal",
+        parse_state: "parsed",
+      },
+      {
+        from_path: "src/index.ts",
+        to: "src/pkg-esm/index.mts",
+        kind: "imports",
+        resolution: "internal",
+        parse_state: "parsed",
+      },
+      {
+        from_path: "src/index.ts",
+        to: "src/cjs.cts",
+        kind: "requires",
+        resolution: "internal",
+        parse_state: "parsed",
+      },
+      {
+        from_path: "src/index.ts",
+        to: "src/pkg-cjs/index.cts",
+        kind: "requires",
+        resolution: "internal",
+        parse_state: "parsed",
+      },
+    ]),
+  );
+});

src/application/ReferenceSourceImportEntries.ts

@@ -157,6 +157,8 @@ const readPackageMains = (
 const RESOLUTION_SUFFIXES: readonly (readonly string[])[] = [
   [""],
   [".ts"],
+  [".mts"],
+  [".cts"],
   [".js"],
   [".mjs"],
   [".cjs"],