Make streaming failures fail closed by default

[Eilen6316]

Adds a fail-closed default for sync and async streaming actions, requires explicit StreamingFailurePolicy.COMMIT_PARTIAL for partial commits, propagates the policy through all streaming entry points, and adds regression tests and documentation. Tests: 396 core tests passed; Graphviz display tests require optional local Graphviz dependencies.

[Eilen6316]

CI note: the documentation workflow concluded with startup_failure before creating any jobs or logs. Recent unrelated PRs show the same startup_failure for .github/workflows/docs.yml. A local Sphinx dirhtml build completed successfully for this branch. The remaining Build Burr, Release Validation, and Build Sphinx Documentation workflows are marked action_required and need maintainer approval for this fork PR.