improvement: cache Daytona client and improve error handling

[hjpinheiro]

Summary

Two improvements to _client.py:

1. Client caching

build_client() created a new Daytona instance (with new HTTP session/connection pool) on every tool invocation. Since the plugin daemon is a long-running process, caching the client by credentials hash avoids redundant connection setup.

# Before — new instance every call:
def build_client(credentials):
    return Daytona(config)

# After — cached per unique credentials:
_client_cache: dict[str, Daytona] = {}

def build_client(credentials):
    key = _cache_key(credentials)
    if key not in _client_cache:
        _client_cache[key] = Daytona(config)
    return _client_cache[key]

The cache key is a SHA-256 hash of api_key + api_url, so different credentials get different clients.

2. Broader error handling in get_sandbox()

Previously only DaytonaNotFoundError was caught. SDK errors like auth failures, rate limits, or server errors leaked as raw exceptions. Now also catches DaytonaError (the parent class) and converts to a descriptive ValueError.

Testing

• py_compile passes
• DaytonaError import verified against daytona==0.187.0