Add watchdogs for hanging runtime paths

[exe-dev-github-integration]

Summary

• add LLM request-open and stream-idle watchdogs
• bound MCP stdio writes/responses and respect cancellation at the MCP tool boundary
• add Python worker default timeout, bounded request writes, stale response filtering, and stderr nulling
• add browser/CDP connect deadline and browser tool outer timeout/cancel guard
• thread real cancellation through compaction
• reject non-regular files in apply_patch/view_image before blocking reads/writes

Verification

• git diff --check
• cargo fmt --check -p browser-use-agent -p browser-use-browser -p browser-use-llm -p browser-use-python-worker
• cargo test -p browser-use-llm -p browser-use-agent -p browser-use-python-worker -p browser-use-browser
• uv run --with pytest python -m pytest -q

Note: branch was rebuilt on rewritten latest main and is one commit over origin/main.

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Summary by cubic

Adds watchdogs and real cancellation across LLM, MCP, browser, and Python worker paths to prevent hangs and surface timeouts clearly. Also tightens file safety in apply_patch and view_image.

• New Features

  • LLM (browser-use-llm): request open timeout (120s) and stream idle timeout (300s) with clear transport errors.
  • MCP stdio (browser-use-agent): bounded notification send, per-request reply timeout, and timed error replies on unknown methods.
  • Browser tool (browser-use-agent, browser-use-browser): outer watchdogs for command/execute/observe/cancel, cancel-aware blocking task, and CDP connect deadline (10s).
  • Python worker (browser-use-python-worker): default tool timeout (120s), bounded request writes with kill/restart on write hang, stale-response filtering, and stderr nulled.
  • Compaction (browser-use-agent): caller cancellation is threaded into summarize with backoff respecting cancel.

• Bug Fixes

  • apply_patch and view_image: reject non-regular files before IO; view_image enforces inline size limit up front.
  • Tests cover connection/idle timeouts, cancel-first behavior, and special-file rejection.

^{Written for commit d948195. Summary will update on new commits.}