fix(data-connect): correctly serialize strings with special characters (#3056)

Replaces manual string escaping with JSON.stringify in DataConnect API client to properly handle newlines and other control characters in mutation arguments.
Adds a regression test for verifying newline escaping.

Fixes #3043

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Yuchen Shi <yuchenshi@google.com>

Author: google-labs-jules[bot]

src/data-connect/data-connect-api-client-internal.ts

@@ -413,10 +413,7 @@ export class DataConnectApiClient {
    */
   private objectToString(data: unknown): string {
     if (typeof data === 'string') {
-      const escapedString = data
-        .replace(/\\/g, '\\\\') // Replace \ with \\
-        .replace(/"/g, '\\"');  // Replace " with \"
-      return `"${escapedString}"`;
+      return JSON.stringify(data);
     }
     if (typeof data === 'number' || typeof data === 'boolean' || data === null) {
       return String(data);

test/unit/data-connect/data-connect-api-client-internal.spec.ts

@@ -928,4 +928,61 @@ describe('DataConnectApiClient CRUD helpers', () => {
         .to.be.rejectedWith(FirebaseDataConnectError, `${serverErrorString}. ${additionalErrorMessageForBulkImport}`);
     });
   });
+
+  describe('String serialization', () => {
+    it('should correctly escape special characters in strings during insert', async () => {
+      const data = {
+        content: 'Line 1\nLine 2',
+      };
+
+      await apiClient.insert(tableName, data);
+      const callArgs = executeGraphqlStub.firstCall.args[0];
+
+      expect(callArgs).to.include(String.raw`content: "Line 1\nLine 2"`);
+    });
+
+    it('should correctly escape backslash', async () => {
+      const data = {
+        content: 'Backslash \\',
+      };
+
+      await apiClient.insert(tableName, data);
+      const callArgs = executeGraphqlStub.firstCall.args[0];
+
+      expect(callArgs).to.include(String.raw`content: "Backslash \\"`);
+    });
+
+    it('should correctly escape double quotes', async () => {
+      const data = {
+        content: 'Quote "test"',
+      };
+
+      await apiClient.insert(tableName, data);
+      const callArgs = executeGraphqlStub.firstCall.args[0];
+
+      expect(callArgs).to.include(String.raw`content: "Quote \"test\""`);
+    });
+
+    it('should correctly escape tab character', async () => {
+      const data = {
+        content: 'Tab\tCharacter',
+      };
+
+      await apiClient.insert(tableName, data);
+      const callArgs = executeGraphqlStub.firstCall.args[0];
+
+      expect(callArgs).to.include(String.raw`content: "Tab\tCharacter"`);
+    });
+
+    it('should correctly handle emojis', async () => {
+      const data = {
+        content: 'Emoji 😊',
+      };
+
+      await apiClient.insert(tableName, data);
+      const callArgs = executeGraphqlStub.firstCall.args[0];
+
+      expect(callArgs).to.include('content: "Emoji 😊"');
+    });
+  });
 });