test(build): harden errors_when_no_cargo_toml against TMPDIR in Rust project #178
Description
Context
The errors_when_no_cargo_toml test in tests/project_root.rs creates a tempdir and asserts that find_project_root returns an error. If TMPDIR is set to a path under a Rust project (possible in some CI environments or nix shells), the walk-up would find an ancestor Cargo.toml and the test would pass unexpectedly.
Impact
Latent test fragility, not a current failure. Standard CI environments and local dev setups are unaffected.
Suggestion
Create a sentinel boundary or assert the found root is within the expected tree if the call succeeds unexpectedly.
Discovered during review of PR #174.