fix(engine): apply WebSearch domain filters

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53257.

Summary

• apply include_domains as site: filters in the DuckDuckGo query
• apply exclude_domains as -site: filters in the DuckDuckGo query
• ignore blank domain entries rather than emitting malformed filters
• reuse the filtered query in the fallback search URL
• add focused unit coverage for include/exclude/blank domain handling

Validation

• cargo +stable fmt --package cortex-engine --check
• cargo +stable test --package cortex-engine tools::handlers::web_search::tests (5 passed)
• cargo +stable check --package cortex-engine --lib
• git diff --check

[greptile-apps]

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