UI/UX - No feedback when flag value exceeds 20000 characters

### How are you running Flagsmith

- [ ] Self Hosted with Docker
- [ ] Self Hosted with Kubernetes
- [x] SaaS at flagsmith.com
- [ ] Some other way (add details in description below)

### Describe the bug

When a user tries to modify a feature value to be more than the limit (20'000 characters), there is no feedback as to why this fails. This is true both with and without change requests and arguably the behavior with change requests on is even worse.

### Steps To Reproduce

See video:
https://github.com/user-attachments/assets/65670396-8a46-4293-9b05-ae1ed0c0199a

### Expected behavior

The user receives feedback that the flag value they are trying to update to/create a change request for exceeds the limit. Ideally with information on how big their flag value is and what the limit is.

### Screenshots

_No response_

Reproduce the issue in the SaaS flag-value editing flow, both with and without change requests, using a value over 20,000 characters. Trace the UI path that handles the failed update or change-request creation. Done means the user is told that the value exceeds the limit, ideally with the current size and the 20,000-character limit.

Written by the indexing model from the issue text.