Fix duplicate exports when two entrypoints share symbols (#26089)

### What does this PR do?

Fixes #5344
Fixes #6356

### How did you verify your code works?

Some test coverage

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Claude Bot <claude-bot@bun.sh>

test/bundler/esbuild/splitting.test.ts

@@ -279,7 +279,6 @@ describe("bundler", () => {
     },
   });
   itBundled("splitting/ReExportESBuildIssue273", {
-    todo: true,
     files: {
       "/a.js": `export const a = { value: 1 }`,
       "/b.js": `export { a } from './a'`,
@@ -609,4 +608,28 @@ describe("bundler", () => {
       stdout: "42 true 42",
     },
   });
+  // Test that CJS modules with dynamic imports to other CJS entry points work correctly
+  // when code splitting causes the dynamically imported module to be in a separate chunk.
+  // The dynamic import should properly unwrap the default export using __toESM.
+  // Regression test for: dynamic import of CJS chunk returns { default: { __esModule, ... } }
+  // and needs .then((m)=>__toESM(m.default)) to unwrap correctly.
+  // Note: __esModule is required because bun optimizes simple CJS to ESM otherwise.
+  itBundled("splitting/CJSDynamicImportOfCJSChunk", {
+    files: {
+      "/main.js": /* js */ `
+        import("./impl.js").then(mod => console.log(mod.foo()));
+      `,
+      "/impl.js": /* js */ `
+        Object.defineProperty(exports, "__esModule", { value: true });
+        exports.foo = () => "success";
+      `,
+    },
+    entryPoints: ["/main.js", "/impl.js"],
+    splitting: true,
+    outdir: "/out",
+    run: {
+      file: "/out/main.js",
+      stdout: "success",
+    },
+  });
 });