fix: migrate Browser Use tool to V4 runs

[MagMueller]

Summary

• use the explicit Browser Use V4 SDK clients
• move sync, async, stream-labelled, and structured tool paths from Tasks to the V4 Runs lifecycle
• preserve the existing smolagents tool interface and add focused sync/async regression coverage

Why

The package-root SDK client still exposes the older tasks resource. Importing the explicit V4 client gives this integration the current runs.create + runs.wait_for_completion lifecycle without changing how Agentic Internet calls its tools.

Validation

• uv run pytest -q (198 passed)
• uv run pytest tests/test_browser_use.py -q (8 passed)
• uv run ruff check agentic_internet/tools/browser_use.py tests/test_browser_use.py
• uv run ruff format --check agentic_internet/tools/browser_use.py tests/test_browser_use.py
• uv run mypy agentic_internet/tools/browser_use.py
• git diff --check
• TruffleHog: zero verified or unverified findings

The repository golden-principles script currently fails on unchanged upstream because docs/exec-plans/active is absent.

[svngoku]

@MagMueller Thanks — merged as bb2dcd0.

Local verification completed successfully: 198 tests passed, and Ruff, formatting, and mypy all passed.

Please follow up on two review findings:

• V4 failed/cancelled runs with no error currently produce success-like responses in the sync, async, and structured tool paths. Guard on the terminal status and add regression coverage.
• stream=True now delegates to the non-stream path, changing the previous streamed-result/steps-summary behavior. Please restore it via V4 event polling or explicitly deprecate/document the change.