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

## 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.

<sup>Written for commit 30fae449d571d3f8a53a3ab9a46360281a49a149.
Summary will update on new commits.</sup>

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