fix(ai): validate AI agent output field names against schema-key constraint (#21834)

## Problem

On a self-hosted instance, an AI Agent workflow action fails at run time
with an opaque model error:

```
The model returned the following errors: tools.0.custom.input_schema.properties:
Property keys should match pattern '^[a-zA-Z0-9_.-]{1,64}$'
```

This is Anthropic's validation on tool `input_schema` **property keys**.
An AI Agent's structured **Output** fields are turned into a JSON schema
and passed to the model as a tool; each output **variable name** becomes
a property key. Anthropic rejects any key that does not match
`^[a-zA-Z0-9_.-]{1,64}$` — most commonly a name containing a **space**
(e.g. `meetings brief`), but also names over 64 characters or with other
symbols.

Until now nothing validated this: `fieldsToSchema` writes
`properties[field.name]` verbatim, so a bad name only failed once the
workflow executed, with an error that gives the user no idea what to
fix. It doesn't reproduce on every instance — it depends purely on how
the workflow's output variables happen to be named.

## Fix

Introduce a single shared check,
`isValidAgentResponseSchemaPropertyKey`, and enforce it in two places:

- **Backend** — `validateAgentResponseFormat` now rejects invalid output
field names at agent **save time** with a clear `userFriendlyMessage`,
instead of letting the broken schema reach the model. This also gates
agents created via the API and re-saves of existing bad data.
- **Frontend** — the output schema builder shows an inline error on the
Variable Name field as soon as an invalid name is entered.

## Tests

- Unit test for the shared validity check (valid + invalid cases:
spaces, leading space, empty, > 64 chars, symbols, unicode).
- Unit test for `validateAgentResponseFormat` covering text/json
formats, valid names, a space in a name, an over-length name, and
reporting multiple invalid names at once.

## Notes for the reporter

The immediate unblock for an affected workflow is to rename the output
variable to remove the space (e.g. `meetings brief` → `meetings_brief`)
and retry the run. With this change the bad name is caught up front with
an explanation rather than failing mid-run.


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packages/twenty-front/src/modules/workflow/workflow-steps/workflow-actions/ai-agent-action/components/WorkflowOutputSchemaBuilder.tsx

@@ -7,6 +7,8 @@ import { InputLabel } from '@/ui/input/components/InputLabel';
 import { styled } from '@linaria/react';
 import { t } from '@lingui/core/macro';
 import { useContext, useState } from 'react';
+import { isNonEmptyString } from '@sniptt/guards';
+import { isValidAgentResponseSchemaPropertyKey } from 'twenty-shared/ai';
 import { IconChevronDown, IconPlus, IconVariable, IconX } from 'twenty-ui/icon';
 import { AnimatedLightIconButton, LightIconButton } from 'twenty-ui/input';
 import { AnimatedExpandableContainer } from 'twenty-ui/layout';
@@ -148,6 +150,17 @@ export const WorkflowOutputSchemaBuilder = ({
     );
   };
 
+  const getVariableNameError = (name: string): string | undefined => {
+    if (
+      !isNonEmptyString(name) ||
+      isValidAgentResponseSchemaPropertyKey(name)
+    ) {
+      return undefined;
+    }
+
+    return t`Use only letters, numbers, underscores, dots or hyphens (max 64 characters).`;
+  };
+
   return (
     <StyledOutputSchemaContainer>
       <InputLabel>{t`Output`}</InputLabel>
@@ -205,6 +218,7 @@ export const WorkflowOutputSchemaBuilder = ({
                         label={t`Variable Name`}
                         placeholder={t`e.g., summary, status, count`}
                         defaultValue={field.name}
+                        error={getVariableNameError(field.name)}
                         onChange={(value) =>
                           updateField(field.id, { name: value.trim() })
                         }