fix: log preflight issues as warnings

[matheusmra]

Context

The PreFlight step is responsible for validating submissions before grading checking for required files and running setup commands (e.g., compilation). When these checks fail, it is expected behavior: the step exists precisely to catch invalid submissions.

However, failures in check_required_files and check_setup_commands were being logged at ERROR level, causing observability tools (e.g., alerting, dashboards) to treat them as application failures. Logs at ERROR should be reserved for unexpected failures in application procedures (e.g., failing to save grading results, failing to fetch a template), not for predictable, student-caused outcomes.

Solution

Downgraded two logger.error calls in PreFlightService to logger.warning:

• check_required_files: a missing required file is a known, expected submission validation failure.
• check_setup_commands: a failed setup command (e.g., compilation error) is a known, expected submission validation failure.

Calls that represent genuine application-level issues were left as logger.error:

• Asset injection failure in PreFlightStep (infrastructure failure).
• Missing sandbox when setup commands are configured (pipeline misconfiguration).
• check_setup_commands called with no sandbox directly via the service (defensive guard, indicates incorrect usage).

Further clarifications

No behavior changes — only log severity is affected. Observability tools will no longer fire false-positive alerts for routine submission validation failures caught by PreFlight.

Related issues

Closes #351

Checklist

• I linked the related issue(s) and explained the motivation.
• I kept this PR focused and scoped to a single concern.
• I added or updated tests for changed behavior (or explained why not needed).
  • No tests needed: log level is an observability concern, not a behavioral one. Existing tests remain valid.
• I ran the relevant tests locally.
• I updated documentation when needed (README/docs/API examples). — N/A
• This PR introduces API contract changes (request/response/endpoint/DTO). — No
• If API changed, I documented compatibility or migration notes. — N/A
• This PR includes breaking changes. — No
• If breaking, I clearly described impact and migration steps. — N/A

[Copilot]

Pull request overview

Adjusts observability for the PreFlight validation path by treating predictable, student-caused validation failures as warnings rather than application errors. This aligns log severity with the pipeline’s intended behavior (PreFlight is expected to reject invalid submissions) and reduces false-positive alerts.

Changes:

• Downgraded logging for missing required submission files from ERROR → WARNING.
• Downgraded logging for failed setup commands (e.g., compile errors) from ERROR → WARNING.

[matheusmra]

@ArthurCRodrigues