fix: Enforce non-zero positive sandbox timeouts to prevent indefinite hangs (#1808)

[Rakshak05]

Description

This PR addresses a vulnerability/bug where a falsy sandbox timeout_seconds value (0, negative, or None) allowed subprocess execution and reader tasks to run indefinitely without a wall-clock limit. If a scanner hung, the process would run forever.

To resolve this:

• Added a @field_validator to SandboxConfig in models.py that raises a ValidationError / ValueError and logs a warning if timeout_seconds is <= 0 or None.
• Added a @field_validator to Settings in config.py that checks both sandbox_timeout and parser_sandbox_timeout_seconds, raising a ValidationError / ValueError and logging a warning if they are <= 0 or None.
• Added accompanying unit tests to verify validation behaviors for both models and settings.

Related Issues

Closes #1808

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Ran the updated unit test suite targeting config settings and sandbox configurations:

python -m pytest testing/backend/unit/test_sandbox_config_model.py testing/backend/unit/test_config_settings.py testing/backend/test_sandbox_executor.py testing/backend/test_sandbox_blocking_issues.py

Results: All 78 tests passed.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

The positive-timeout validation is narrowly scoped, but the branch is behind main. Please rebase and add a regression test through the actual task/sandbox configuration path, not only model/settings tests, then rerun CI.

[utksh1]

Please add coverage for the execution path that previously accepted 0/None, not only model validation. Confirm that environment/config parsing cannot silently create an unbounded timeout, then rebase on main.