fix: apply web search domain filters

[Bortlesboat]

Fixes PlatformNetwork/bounty-challenge#53257.

What changed:

• WebSearch now appends include_domains as site: filters to the DuckDuckGo query.
• WebSearch now appends exclude_domains as -site: filters to the DuckDuckGo query.
• Existing mutual-exclusion validation for include/exclude domain filters is preserved.
• Added focused regression tests for include and exclude domain URL construction.

Verification:

• cargo test -p cortex-engine include_domains_are_added_as_site_filters_to_search_url -- --nocapture (red before fix: missing helper)
• cargo test -p cortex-engine domain -- --nocapture
• cargo check -p cortex-engine
• cargo fmt --check --package cortex-engine
• git diff --check

[greptile-apps]

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