fix(workflow): serialize object variables in resolved prompts (#21612)

## Problem

When a workflow passes a variable into a text input (e.g. an **AI
Agent** prompt) and that variable resolves to an object or array, the
resolved string contained `[object Object]` instead of the actual
content. The AI then received useless input.

## Cause

`resolveString` in the shared variable resolver builds the final string
with `String.prototype.replace`. When an embedded `{{variable}}`
resolved to an object, the replace callback returned the object
directly, which JS coerces to `"[object Object]"`. The rich-text
resolver had the same issue via `String(resolvedValue)`.

## Fix

When an embedded variable resolves to a non-null object (or array),
serialize it with `JSON.stringify` before inserting it into the
surrounding string. Primitive values keep their existing coercion
behavior, and the single-variable case (`{{message}}` with nothing
around it) still returns the raw object so non-string consumers are
unaffected.

Applied the same guard to both the plain and rich-text variable
resolvers for consistency.

## Tests

Added cases covering embedded object/array variables in both resolvers,
plus a guard test confirming a standalone `{{variable}}` still returns
the raw object.

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packages/twenty-shared/src/utils/__tests__/rich-text-variable-resolver.test.ts

@@ -126,6 +126,23 @@ describe('resolveRichTextVariables', () => {
     );
   });
 
+  it('should serialize object values as JSON instead of [object Object]', () => {
+    const contextWithObject = {
+      step1: {
+        message: { foo: 'bar', count: 1 },
+      },
+    };
+
+    const input =
+      '[{"type":"paragraph","content":[{"type":"variableTag","attrs":{"variable":"{{step1.message}}"}}]}]';
+
+    const result = resolveRichTextVariables(input, contextWithObject);
+
+    expect(result).toBe(
+      '[{"type":"paragraph","content":[{"type":"text","text":"{\\"foo\\":\\"bar\\",\\"count\\":1}"}]}]',
+    );
+  });
+
   it('should preserve regular {{variable}} patterns in non-variableTag contexts', () => {
     const input =
       '[{"type":"paragraph","content":[{"type":"text","text":"Regular {{step1.message}} pattern"}]}]';

packages/twenty-shared/src/utils/__tests__/variable-resolver.test.ts

@@ -94,6 +94,27 @@ describe('resolveInput', () => {
     expect(resolveInput(input, context)).toEqual(expected);
   });
 
+  it('should serialize an object variable embedded in a string', () => {
+    expect(resolveInput('Log this message: {{user}}', context)).toBe(
+      'Log this message: {"name":"John Doe","age":30}',
+    );
+  });
+
+  it('should serialize an array variable embedded in a string', () => {
+    expect(
+      resolveInput('Themes: {{preferences}}', {
+        preferences: ['dark', 'light'],
+      }),
+    ).toBe('Themes: ["dark","light"]');
+  });
+
+  it('should return the raw object when the whole string is a single variable', () => {
+    expect(resolveInput('{{user}}', context)).toEqual({
+      name: 'John Doe',
+      age: 30,
+    });
+  });
+
   it('does not wrap string variables with double quotes', () => {
     expect(
       resolveInput('{ {{test}}: 2 }', {

packages/twenty-shared/src/utils/rich-text-variable-resolver.ts

@@ -41,7 +41,11 @@ export const resolveRichTextVariables = (
     (_, variableTypeFirst: string, variableAttrsFirst: string) => {
       const variable = variableTypeFirst ?? variableAttrsFirst;
       const resolvedValue = evalFromContext(variable, context);
-      const textValue = isDefined(resolvedValue) ? String(resolvedValue) : '';
+      const textValue = !isDefined(resolvedValue)
+        ? ''
+        : typeof resolvedValue === 'object'
+          ? JSON.stringify(resolvedValue)
+          : String(resolvedValue);
 
       return buildTextNodesWithLineBreaks(textValue);
     },

packages/twenty-shared/src/utils/variable-resolver.ts

@@ -78,6 +78,10 @@ const resolveString = (
   return input.replace(VARIABLE_PATTERN, (matchedToken, _) => {
     const processedToken = evalFromContext(matchedToken, context);
 
+    if (typeof processedToken === 'object' && processedToken !== null) {
+      return JSON.stringify(processedToken);
+    }
+
     return processedToken;
   });
 };