test: expand DevPath Sentinel validator test coverage

[jyotish6699]

Summary

Closes #1916

Expand the automated test coverage for DevPath Sentinel's existing validators with additional edge-case and combined-validation scenarios.

This PR focuses entirely on test coverage and does not change the existing validator behavior.

Changes

Dataset Validator

Expanded tests/test_sentinel_dataset_validator.py to cover:

• Multiple duplicate project IDs
• Multiple duplicate project titles
• Whitespace-only required fields
• Empty required list fields
• Multiple missing required fields
• Multiple validation failures occurring together
• Deterministic ordering of duplicate validation results

The Dataset Validator test suite now contains 15 tests.

Starter Code Integrity Validator

Expanded tests/test_starter_code_validator.py to cover:

• Nested starter code files
• Nested orphan starter code files
• Multiple empty starter code files
• Multiple hidden files
• Multiple unsupported file extensions
• All configured supported starter code extensions
• Invalid dataset structures where the JSON root is not a project list

The Starter Code Integrity Validator test suite now contains 15 tests.

Test Coverage

The Sentinel validator test suite now contains:

• 15 Dataset Validator tests
• 15 Starter Code Integrity Validator tests
• 30 Sentinel validator tests in total

All Sentinel validator tests pass successfully.

Scope

This PR:

• Improves regression coverage
• Covers important edge cases
• Verifies combined validation scenarios
• Preserves the existing validator implementation
• Follows the existing pytest structure and helper patterns

No production validator behavior has been changed.

Testing

pytest --noconftest tests/test_sentinel_dataset_validator.py tests/test_starter_code_validator.py -v

[vercel]

@jyotish6699 is attempting to deploy a commit to the komalsony234-1530's projects Team on Vercel.

A member of the Team first needs to authorize it.

[jyotish6699]

The DevPath CI / build check is failing during test collection before the Sentinel tests run.

The failure is caused by src/utils/portfolio_analyzer.py using re.escape() and re.compile() without importing the re module.

This issue is unrelated to the changes in this PR, which only expand DevPath Sentinel test coverage.