Validate task status query parameter

[HitanshiThakar]

Description

Added validation for the optional status query parameter in GET /api/v1/tasks.

Previously, invalid status values returned an empty result set. The endpoint now validates the value against the supported TaskStatus enum values and returns a structured 400 Bad Request response for invalid inputs.

Also added tests covering both valid and invalid status filters.

Related Issues

Closes #467

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?

pytest testing/backend/test_task_pagination.py
Result:
12 test 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]

Looks good. The status query parameter is validated against TaskStatus and invalid values return a clear 400 response with test coverage. CI is passing.