test: pin the fence read-error path-strip with a positive unit test #4191

Context

From the review of #4127 (head 2fb24a89), commit fix(coordination): stabilize fence read errors:

loadLegacyCoordinationWriterFence now strips the trailing quoted absolute path from read-failure reason (legacy_writer_fence.ts:145-152):

const path = (error as NodeJS.ErrnoException).path;
const reason = error instanceof Error ? error.message : "legacy writer fence read failed";
// ...
reason: typeof path === "string" ? reason.replace(` '${path}'`, "") : reason,

Problem

The strip is a string-suffix match against Node's current fs error message format. The parity fixture (tests/fixtures/control_plane/legacy_writer_fence_caller_parity_v0.json:136) pins the path-free expectation, but nothing constructs a real fs error carrying error.path and asserts the strip actually fired. If Node ever changes its message shape (unquoted path, double quotes, path mid-message), the strip silently stops working and absolute local paths leak back into public reason — with no test going red.

Suggested fix

One focused unit test: construct an ENOENT-shaped error whose .path is a temp absolute path and whose .message ends with '<path>'; call the reader; assert reason contains no path fragment. A negative arm (error without path) asserting the message is preserved unchanged would also cover the defensive branch.

Non-blocking follow-up from #4127 (comment).