test: isolate OS managed config writes

[Edwinhe03]

Summary

• make the pytest suite fail closed before any unmocked OS-managed config write can reach the sudo-backed writer
• redirect the managed-resolution persistence tests to a temporary managed-settings.json
• retain coverage that both user-level and machine-wide Claude settings receive the managed model

Root cause

TestStateFileIsNotRewritten uses a fake workspace together with a managed manifest that enables use_as_global_settings. Its fixture redirected state.json and the user-level Claude settings file, but not Claude Code's OS-managed settings path or privileged writer.

Calling claude.write_tool_config therefore reached the real /etc/claude-code/managed-settings.json. On a developer machine where sudo was available, running the test suite could merge the fake workspace routing and auth configuration into the real machine-wide file.

Fix

• patch managed_files._sudo_replace in the suite-wide isolation fixture to fail any test that reaches an unmocked privileged write
• give the affected persistence fixture a temporary managed-settings path and a local writer
• assert the temporary managed file still receives the expected model, so the production behavior remains covered

Production behavior is unchanged: ucode can still write the actual OS-managed config when an administrator explicitly enables global settings.

Validation

• uv run pytest tests/test_managed_resolve.py tests/test_managed_files.py tests/test_agent_claude.py::TestWriteToolConfigManagedSettings tests/test_agent_codex.py::TestCodexManagedConfig — 86 passed
• uv run ruff check tests/conftest.py tests/test_managed_resolve.py
• git diff --check
• verified the real /etc/claude-code/managed-settings.json checksum and mtime remain unchanged during the focused suite

Full-suite verification

• uv run pytest — 1988 passed, 39 skipped, 1 environment-specific failure
• the remaining failure is TestCliWiring.test_successful_apply_exits_zero, where the bootstrap installer refuses to overwrite this host's existing /usr/local/bin/databricks; it does not exercise managed config writes
• /etc/claude-code/managed-settings.json remained byte-for-byte identical before and after the suite (sha256: 7544dc52fb73317e3699b30724e81b46f82de8bac4a360e88cfcbda0b79d253a), with unchanged mtime, ownership, mode, size, and file attributes