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/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;
   });
 };