[Fix] Daytona read_file returns FileNotFoundError for missing files

[CREDO23]

Summary

• Map Daytona's FILE_NOT_FOUND failures in read_file to FileNotFoundError, matching the SandboxSession contract OpenSandbox already fulfills — a missing receipt no longer escapes as a raw DaytonaError and crashes the stream on the first verification
• Re-raise all other DaytonaErrors unchanged

Test plan

• uv run pytest tests/unit/sandbox/test_daytona_provider.py

High-level PR Summary

This PR fixes a bug in the Daytona sandbox provider where read_file operations for missing files were raising raw DaytonaError exceptions instead of the expected FileNotFoundError. The fix introduces a helper function _is_not_found that detects file-not-found conditions from Daytona errors (checking for DaytonaNotFoundError subclass, 404 status codes, and FILE_NOT_FOUND in the error message) and properly translates them to FileNotFoundError to match the SandboxSession contract. This prevents crashes during receipt verification when files are missing.

⏱️ Estimated Review Time: 5-15 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_backend/tests/unit/sandbox/test_daytona_provider.py
2	surfsense_backend/app/sandbox/providers/daytona.py

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