Client is using outdated header 'anthropic-beta': 'prompt-caching-2024-07-31'

Inspected `claudette.Client??` on the SolveIt platform today and saw:
```python
class Client:
def __init__(self, model, cli=None, log=False, cache=False):
"Basic Anthropic messages client."
self.model,self.use = model,usage()
self.text_only = model in text_only_models
self.log = [] if log else None
self.c = (cli or Anthropic(default_headers={'anthropic-beta': 'prompt-caching-2024-07-31'}))
self.cache = cache
```

I would assume it is no longer beta after 1 year of introduction?

Start at the reported Client.__init__ entry point and inspect how Anthropic(default_headers=...) is configured. Check the current Anthropic prompt-caching header status, then verify that client initialization no longer relies on an outdated beta header and that the existing cache path still works.

Written by the indexing model from the issue text.