Enable Web Search support in Groundedness evaluator

[mmkawale]

Summary

• Add web_search to groundedness restricted-tool allowlist
• Add groundedness behavior test that expects Web Search to pass

Validation

• Ran targeted tests:
  • python -m pytest assets/evaluators/tests/test_evaluators_behavior/test_groundedness_evaluator_behavior.py -k "web_search or openapi or bing_grounding or azure_ai_search" -q
• Result: 4 passed

Formatting

• Ran Black in check mode on changed files.
• Black reports formatting differences, but this PR intentionally keeps whitespace churn minimal.