#2828 feat(sdk): expose Browserbase search and fetch

Summary

• expose Browserbase Search and Fetch through the TypeScript and Python browserbase facades
• add equivalent SearchBrowserbase and FetchBrowserbase APIs to Go using the existing Browserbase HTTP transport
• normalize response models across SDKs and document usage

Testing

• pnpm --filter @browserbasehq/stagehand build
• pnpm --filter @browserbasehq/stagehand test:unit (199 passed)
• Python Ruff, ty, and pytest (460 passed, 1 skipped)
• Go package tests (passed)

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Summary by cubic

Implements AP-2921 by exposing Browserbase Search and Fetch through the TypeScript and Python browserbase facades and Go's SearchBrowserbase/FetchBrowserbase functions, without launching a browser.

• Uses the official Browserbase SDK in TypeScript and Python and the existing HTTP transport in Go.
• Validates query limits, fetch formats, schema requirements, URLs, and required Go search-result fields.
• Normalizes result models, adds typed fetch content handling in Go, and keeps the three SDKs' APIs in sync.
• Adds README examples and tests, and improves Go extension drift checks with SHA-256 hashes and cache invalidation.