Improve Python SDK linting rules and upgrade Ruff

[antonvishal]

why

Tighten linting rules for packages/sdk-python to catch common bugs (mutable defaults, unchained exceptions, unescaped regexes) early.

what changed

• Upgraded ruff to 0.16.8 and updated uv.lock.
• Enabled high-value lint rules:
  • B: mutable default arguments (B006), useless comparisons/expressions (B015, B018), explicit exception chaining (B904).
  • C4 (comprehensions), PGH (strict type ignore comments).
  • RUF: sorted __all__/__slots__ (RUF022, RUF023), raw string regexes (RUF039), and stale noqa cleanup (RUF100).
  • T10 (debugger breakpoints), W (pycodestyle warnings), and external = ["S"].
• Fixed existing violations:
  • Sorted __all__ in src/stagehand/__init__.py and __slots__ in StagehandBrowser.
  • Added explicit exception chaining (raise cleanup_error from error).
  • Converted regex pattern strings in unit tests to raw strings (r"^...").

test plan

• uv run pytest: 562 passed, 1 skipped
• uv run ruff check .: all checks passed
• uv run ruff format --check .: 58 files checked, no formatting changes needed
• uv run ty check: all checks passed
• uv lock --check: passed

---

Summary by cubic

Tightens linting for packages/sdk-python so common bug patterns are caught in CI, and upgrades ruff to 0.16.8.

• Adds rules for mutable defaults, useless comparisons/expressions, exception chaining, comprehensions, raw regexes, breakpoints, pycodestyle warnings, and sorted __all__/__slots__.
• Fixes existing violations by sorting __all__ and __slots__, chaining cleanup_error from the original error, and converting test regex patterns to raw strings.
• Sets external = ["S"] so Bandit security rules are excluded from the new lint set.
• Updates uv.lock and confirms pytest, ruff, ty, and uv lock --check all pass.

^{Written for commit 9e6d361. Summary will update on new commits.}

[changeset-bot]

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Latest commit: 9e6d361

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[github-actions]

This PR is from an external contributor and must be approved by a stagehand team member with write access before CI can run.
Approving the latest commit mirrors it into an internal PR owned by the approver.
If new commits are pushed later, the internal PR stays open but is marked stale until someone approves the latest external commit and refreshes it.

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	pypi/​ruff@​0.15.7 ⏵ 0.16.8	+1

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