feat: add SerpApi websearch tool

[Wang-tianhao]

Summary

• add a new built-in websearch tool backed by SerpApi with google_light as the default mode and google as the richer standard mode
• wire the tool through the domain catalog, executor, service container, output formatting, permission handling, and built-in agent tool lists
• add normalized XML output, parser/service tests, catalog tests, and snapshots for the new tool

Verification

• cargo check
• cargo test -p forge_services websearch
• cargo test -p forge_domain websearch
• INSTA_UPDATE=always cargo test -p forge_app test_web_search

Co-Authored-By: ForgeCode noreply@forgecode.dev

[CLAassistant]

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