Sdk improvements

[bchatelard]

• SandboxServiceError — New typed exception for HTTP 5xx errors from the sandbox executor, with status_code attribute. Raised from executor_client.py instead of leaking raw httpx.HTTPStatusError.
• SandboxTimeoutError — httpx.TimeoutException (including ReadTimeout) is now converted to SandboxTimeoutError in both buffered and streaming requests.
• exec.py simplified — Removed the broad except Exception blocks that were silently wrapping errors into CommandResult. Errors now propagate as typed exceptions instead of masquerading as failed commands.
• filesystem.py cleanup — Fixed isinstance() anti-pattern in exception handling; SandboxServiceError and SandboxFilesystemError are now caught with proper except clauses.
• executor_client.py streaming fix — Fixed ResponseNotRead crash when accessing response.text on streaming responses (need response.read() / await response.aread() first).
• close() / del fix — Used getattr(self, "_closed", True) to prevent AttributeError when init fails partway through (sync + async clients).